The problem as I understand it is a combining of roles.. not confounding or mixing but fusing the roles into something different. The role of the priest doesn't seem to match the role of the Presbyter. But if they are the same, then why the difference.
μαρτυρεί γαρ ότι συ ιερεύς εις τον αιώνα κατά την τάξιν Μελχισεδέκ
Contestatur enim : Quoniam tu es sacerdos in æternum, secundum ordinem Melchisedech.
For he testifieth: Thou art a priest for ever according to the order of Melchisedech.
The Vulgate regularly renders Greek πρεσβύτερος ["old man"], πρεσβύτεροι by Latin senior [from senex], (plural) seniores ["ancients"], except in Acts 20:17, 22:5, where it has majores natu ["ancients"], and in Acts 14:22, 15:2, 1 Timothy 5:17, 19, Titus 1:5, James 5:14, where the Greek form is retained as presbyter, presbyteri ["priests"]. The same men who in Acts 15:2 are called apostoli et presbyteri ["apostles and priests"], are called in verses 4, 6, 22, 23 apostoli et seniores ["apostles and ancients"]; the Greek having uniformly ἀπόστολοι και πρεσβύτεροι.
It is both a conflation of words and of roles.
This word [translated as PRIEST] (etymologically "elder", from presbyteros, presbyter) has taken the meaning of "sacerdos", from which no substantive has been formed in various modern languages (English, French, German). The priest is the minister of Divine worship, and especially of the highest act of worship, sacrifice. New Advent
So admitadly, there was a diverging from what had taken place. That diverging accompanied the role of a sacerdotal priest in the Catholic Church. Originally, the ELDER (πρεσβύτερος presbyteros) was in a leadership role for each location of the Body of Christ. It had nothing to do with a separate role of some who believer they offer a same repeating sacrifice day after day.
(3) in the Christian churches, those who, being raised up and qualified by the work of the Holy Spirit, were appointed to have the spiritual care of, and to exercise oversight over, the churches. To these the term "bishops," episkopoi, or "overseers," is applied (see Acts 20, ver. 17 with ver. 28, and Tts 1:5, 7), the latter term indicating the nature of their work, presbuteroi their maturity of spirtual experience. The Divine arrangement seen throughout the NT was for a plurality of these to be appointed in each church, Act 14:23; 20:17; Phl 1:1; 1Ti 5:17; Tts 1:5. The duty of "elders" is described by the verb episkopeo. They were appointed according as they had given evidence of fulfilling the Divine qualifications, Tts 1:6-9; cp. 1Ti 3:1-7; 1Pe 5:2; Vines Expository Dictionary of New Testament Words
For the Body of Christ, all believers are priests who offer spiritual sacrifices. Ironically, it is Peter who tells us this.
Ye also, as lively stones, are built up a spiritual house, an holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ. 1 Peter 2:5
So, the Catholic word translated as PRIEST came from PRESBYTER that orginally (and still to some) meant ELDER whose role was to study, teach, lead. At that time, all believers were of the same priesthood as Christ who is our High Priest. Once the word PRESBYTER became unhinged from its New Testament context, the newly redefined PRESBYTER role became one of a like Old Testament PRIEST who was of continually physical sacrificing.