“elder” is a poor translation for “presbyteros” simply because age is not a requirement for the priestly office. Apostle John, for example, was a teenager yet the command “do this in memorial of me” was directed at him as well. The root of “presbyteros” is the same as the quizzical “presbeia” and if we were to translate “presbyteros” today into English we probably could use “leader”. However, we already have a direct borrow from Greek and we do not need to translate “presbyteros” at all. “Priest” is a contraction from “presbyteros” that went through centuries of the English hideous spelling and pronunciation grinder.
That “bishop” and “priest” were used interchangeably in the early church I have no dispute. First, bishop even today is a subspecies of priest. Second, in a small local church a bishop may not require to delegate his duty as homilist and eucharistic minister to anyone.
It is also true that Christ is the only priest ontologically speaking, and the sacrifice He is offering is He Himself. But Christ did command others to “do it in memorial of [Him]”. So it is the same sacrifice of the hill of golgotha that the priest makes availablel to us at Mass. Compare similar delegation done by Christ to absolve sins in John 20.
I think elder is understood to mean spiritual maturity.