PLEASE clarify that. Jesus spoke by his own mouth saying that the Holy Spirit would guide us into all Truth. You can't be implying that He's not infallible. You cannot possibly be opining that He needs help in giving that guidance. At least, you cannot be doing either of those things without being heretical, which I doubt is the case. So, more clarification of your precise meaning is needed.
Regarding the priesthood, I recommend a thorough reading of the New Testament book of Hebrews; especially chs 5-10 where the import and implications of Jesus' designation as High Priest in the Order of Melchizedek are laid out.
The writer of Hebrews, lays out the orthodox Christian explanation of who Jesus is, and what he accomplished, in a way that would be immediately understood by members of the Judaic culture, which is to be expected because Jesus life and work fulfilled and retired the requirements of the Old Covenant, replacing by his perfect sacrifice that way of imperfect sacrifices, and opening for us by his own flesh upon the cross, a new and living way, sanctified in His own blood.
As the writer says, "having boldness to enter into the holiest place by the blood of Jesus...let us draw near."
In the context, this drawing near is as the Old Testament High Priest alone could once, annually, draw near to the Ark of The Covenant, to appear before The LORD Who sat enthroned between the two cherubim on the atonement cover and offer the required sacrifices in dry-mouthed sobriety, and with trembling.
In teh same way, now, owing to Christ's position as High Priest in the Order of Melchizedek we may also draw near, but with boldness.
And THIS is the wonder of the gospel; that all the world of sinful man has been granted free and open access to that Holiest Place by God's gracious sacrifice of the Only Sufficient Mediator.
Gladly. And no, I'm not implying that the Holy Ghost needs help.
Your assumption here is that Jesus is saying that the Holy Spirit will lead *believers* to all truth *as individuals*. But neither the notion of believers in general, nor of individuals is present in the text. The Greek text has "humas" = "you plural" there. He will lead "you all" into all truth. Now who exactly is that "you all"?
Well, look at the whole context of John 16--it is addressed to the Apostles, not to all believers. You can tell that because of all the language he is using which is specific to the Apostles and not to you or I 2000 years later: "they will expel you from the synagogues" "I did not tell you this from the beginning", "I have much more to tell you, but you cannot bear it now". In other words, Jesus is speaking directly to the Apostles that saying that the Holy Spirit will come and lead YOU ALL to all truth, *i.e. the Apostles as a group* whom Christ is speaking to.
This more restricted reading fits the whole context better. It also makes more sense. Otherwise, how is it that two believers--supposely both with the same access to the Holy Spirit--come out with two different interpretations?
And I've read Hebrews minutely! To wit...on Melchizedek, let me ask you....what does it *mean* for Christ to be a priest "of the order of Melchizedek"? Melchizedek didn't die on a cross. Melchizedek didn't offer animal sacrifices. So what was it that he offered? And how does Christ share the same kind of priesthood??