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Having Saint So-and-so's pinky toe buried under an altar was necessary?

You answered your own question. After the Second Council of Nicaea it became a legal obligation; doesn't mean it was not a custom all the time before that. As your lengthy quote form Wikipedia shows, veneration of relics is a custom that predates Christianity.

More relevant to us is what the Early Church taught.

Few points of faith can be more satisfactorily traced back to the earliest ages of Christianity than the veneration of relics. The classical instance is to be found in the letter written by the inhabitants of Smyrna, about 156, describing the death of St. Polycarp. After he had been burnt at the stake, we are told that his faithful disciples wished to carry off his remains, but the Jews urged the Roman officer to refuse his consent for fear that the Christians "would only abandon the Crucified One and begin to worship this man". Eventually, however, as the Smyrnaeans say, "we took up his bones, which are more valuable than precious stones and finer than refined gold, and laid them in a suitable place, where the Lord will permit us to gather ourselves together, as we are able, in gladness and joy, and to celebrate the birthday of his martyrdom." This is the keynote which is echoed in a multitude of similar passages...
Relics

Curiously, in the Apostolic constitutions (ca AD 400) we find admonition against girly squeamishness such as yours:

Section 6 [Conclusion] of the Work
That It is the Custom of Jews and Gentiles to Observe Natural Purgations, and to Abominate the Remains of the Dead; But that All This is Contrary to Christianity.
XXX . Do not therefore keep any such observances about legal and natural purgations, as thinking you are defiled by them. Neither do you seek after Jewish separations, or perpetual washings, or purifications upon the touch of a dead body. But without such observations assemble in the dormitories, reading the holy books, and singing for the martyrs which are fallen asleep, and for all the saints from the beginning of the world, and for your brethren that are asleep in the Lord, and offer the acceptable Eucharist, the representation of the royal body of Christ, both in your churches and in the dormitories; and in the funerals of the departed, accompany them with singing, if they were faithful in Christ. For precious in the sight of the Lord is the death of His saints. And again: O my soul, return unto your rest, for the Lord has done you good. And elsewhere: The memory of the just is with encomiums (Proverbs 10:7). And, The souls of the righteous are in the hands of God (Wisdom 3:1). For those that have believed in God, although they are asleep, are not dead. For our Saviour says to the Sadducees: But concerning the resurrection of the dead, have you not read that which is written, I am the God of Abraham, and the God of Isaac, and the God of Jacob? God, therefore, is not the God of the dead, but of the living; for all live to Him (Exodus 3:6; Luke 20:38). Wherefore, of those that live with God, even their very relics are not without honour. For even Elisha the prophet, after he was fallen asleep, raised up a dead man who was slain by the pirates of Syria. For his body touched the bones of Elisha, and he arose and revived. Now this would not have happened unless the body of Elisha were holy. And chaste Joseph embraced Jacob after he was dead upon his bed; and Moses and Joshua the son of Nun carried away the relics of Joseph, and did not esteem it a defilement. Whence you also, O bishops, and the rest, who without such observances touch the departed, ought not to think yourselves defiled. Nor abhor the relics of such persons, but avoid such observances, for they are foolish. And adorn yourselves with holiness and chastity, that you may become partakers of immortality, and partners of the kingdom of God, and may receive the promise of God, and may rest for ever, through Jesus Christ our Saviour.
Apostolic Constitutions (Book VI)

Use of relics was not solely a "Christian" thing, though

Indeed. It is normal element of veneration of anyone held holy. So perhaps, the world should pay less attention to what Protestant vandals find "macabre" and "creepy" and instead worship God properly?

That this council mandated relics placed underneath altars of all Catholic churches in the eighth century in no way makes such actions nor the requirement of such a "Christian" practice. The Catholic church mandated many things that were nowhere found within the early church of the Apostles' lives nor of that even a few centuries later. What we DO know is that, even in the Apostles' days, there were wolves creeping in to scatter the flock. That superstitions crept in along with them should be no surprise. It is beyond question that the early church did NOT call upon martyred saints, nor worship them, much less presume their remains were imbued with supernatural powers so that they had to be preserved under altars. Worldly pagan ideals crept into the church and were absorbed in the idea that more souls could be converted. It didn't take long before many of the pagan practices became absorbed and then embedded too deeply to be purified out of Christianity.

We know from the book of Acts that neither St. Stephen, the first Christian martyr, nor St. James, who was killed by Herod, were invoked by the Apostolic church in any manner, because, if they had been, there would have been mention of such an important matter seeing as other events of less importance were mentioned. If such a practice was in conformity with the Apostolic doctrine, it would have been brought forward in the epistles of St. Paul and the other Apostles, yet there is nothing said about it. Even in the writings of the early church fathers such as Clement and Polycarp, there is nothing spoken about invoking the aid or intercession of the saints. It is always to God that believers are encouraged to pray to so that, if they are found in the same circumstances as the martyrs, God may grant them faith, peace, patience, wisdom and strength. So we know that this was a later innovation and not something taught by the Apostles.

It's interesting that you mention the martyrdom of Polycarp because the believers of whom he was bishop, rather than take up his remains to venerate, said in an epistle which was issued by the church of Smryna states:

or worshop anyone else. "We worship,"

"Jesus Christ, who is the Son of God

we love them, as they deserve it, on account of the unconquerable love which they had for their Master and King; and would to God that we should become their disciples and partakers of their zeal."

A Treatise on Relics

Even in the fourth century, the orthodox Christians considered worship of every created being as idolatry. Polycarp's remains were treated respectfully and buried and the date of his martyrdom marked and remembered for the edification of other Christians. It did not mean, however, that they gathered his bones to venerate or to place them somewhere where he could protect them or provide some kind of power.

In the same book, Calvin had this to say about Marian devotion and relics:

The Blessed Virgin.— The belief that the body of the Virgin was not interred on earth, but was taken to heaven, has deprived them of all pretext for manufacturing any relics of her remains, which otherwise might have been sufficiently abundant to fill a whole churchyard; yet in order to have at least something belonging to her, they sought to indemnify themselves for the absence of other relics with the possession of her hair and her milk. The hair is shown in several churches at Rome, and at Salvatierra in Spain, at Maçon, St Flour, Cluny, Nevers, and in many other towns. With regard to the milk, there is not perhaps a town, a convent, or nunnery, where it is not shown in large or small quantities. Indeed, had the Virgin been a wet-nurse her whole life, or a dairy, she could not have produced more than is shown as hers in various parts. How they obtained all this milk they do not say, and it is superfluous here to remark that there is no foundation in the Gospels for these foolish and blasphemous extravagances.

This http://beggarsallreformation.blogspot.com/search?q=relics article goes into further detail about the many ways faithful Christians have been hoodwinked and fooled out of their money and, sadly, of their faith, when scams are discovered. So-called relic "miracles" were counterfeited and the devil laughed at the gullibility and vain seeking for signs. Calvin puts it well:

"I admit that people do not arrive at once at open idolatry, but they gradually advance from one abuse to another until they fall into this extremity, and, indeed, those who call themselves Christians have, in this respect, idolatrised as much as Pagans ever did. They have prostrated themselves, and knelt before relics, just as if they were worshiping God; they have burnt candles before them in sign of homage; they have placed their confidence in them, and have prayed to them, as if the virtue and the grace of God had entered into them. Now, if idolatry be nothing else than the transfer elsewhere of the honour which is due to God, can it be denied that this is idolatry? This cannot be excused by pretending that it was only the improper zeal of some idiots or foolish women, for it was a general custom approved by those who had the government of the church, and who had even placed the bones of the dead and other relics on the high altar, in the greatest and most prominent places, in order that they should be worshiped with more certainty." -John Calvin, A Treatise on Relics

You may want to read the above document further as it speaks about the origin of the veneration of relics and the superstitions, legends and myths that people who are actually weak in faith need to cling to for strength rather than through faith in Christ - who rewards those who diligently seek him and says that they are blessed who believe even when they don't see. It is not "girlish squeamishness" that compels me to speak, but a hatred of the false gospel that snares the unknowing into believing lies based upon tricks. We are better served by heeding the sacred Scriptures than the rationalizations of men.