No difference between “mortal sin” and “sin”.
Actually there is and it is part of Apostolic Tradition found in both the Sacred Scriptures and Tradition
In Matthews Gospel 12:31-32 it is clear that there are differences of sin. Therefore I tell you that every sin and blasphemy will be forgiven men but the blasphemy against the Holy Spirit will not be forgiven. And whoever says a word against the Son of man will be forgiven but whoever speaks against the Holy Spirit will not be forgiven either in this age or the age to come.
So it is clear in this passage alone there is a sin which can never be forgiven, which Catholic Theologians say is one who never confesses their sin and repents and dies that way (blasphemy against the Holy Spirit] but even in this passage some sins can be forgiven in age to come [early Church Fathers saw this as postmortem purification before one can participate in the Beatific vision where they enjoy perfect communion with the Triune God and the all the angels and saints, i.e. purgatory]
1 John 5:16-17 clearly points to differences in sin. There is sin that is deadly and while all wrongdoing is sin, but there is sin which is not deadly.
Well it is obvious that the sin that is not deadly is not the unforgivable sin described in MT 12:31-32. To further illustrate various types of sin found in the NT, we can also refer to James 5:13-19 where the Apostle James speaks of both anointing the sick and confession [both Sacraments in the Apostolic Churches, i.e. Catholic, Orthodox, Oriental Orthodox and Assyrian Church of the East]. We read on that if says if someone brings back someone who has wondered away from the truth, that will save the soul of the person who was brought back and that act of charity will cover a “multitude of sins”. Notice the act of bring one back covers a multitude of sins, not “all sins” and certainly not the “unforgivable sin”
Saint Peter in his 1st Epistle [4:8] has a theology of sin that is consistent with Saint James and illustrates differences in types of sin. For Saint Peter writes “Above all hold unfailing love for one another since love covers a multitude of sins.
2 OT books in the Canon of all Apostolic Churches, mentioned above already, have this theology of people acting in charity and covering sins. You can see this in both Tobit 12:9 and Sirach 29:12.
Finally, in Saint Paul’s letter to the Romans [5:12] we read all men sinned. Again, this has to be interpreted in context. This is referring to Adam and thus “original sin” not personal sin of anyone. Thus, by all sinned it does not mean for example a child of 1 or 2 years old committed “personal sin” that would be heretical nonsense. Thus, in this passage when Saint Paul speaks of “all have sinned” it does not mean all have actually committed a personal sin, venial or mortal, in Catholic theology.
In summary, the views you posit are the views of those protestants from north of the Alps starting in the 16th century and have not basis in the Apostolic Church and those Churches that existed in the Roman world south of the alps in Europe (Italy and Greece] and the Eastern part of the Roman empire and most importantly it is not supported by Sacred Scripture.