This is an extensive topic of discussion, so if another thread is started on it, that would probably better so we don’t veer too far away from the OP subject.
Briefly I’ll just put it this way. There’s a completely different understanding of sin between the Catholic and Orthodox world. This in and of itself will certainly play a large role not only on whether or not the sin of adultery is committed, but also how and when the sin is forgiven if deemed adultery.
The reason why the Catholic teaching appeals to me is that it seems to err on the side of caution. From what Jesus has said, we know adultery can be committed even if you’re not married. Sexual lust in your thoughts and heart can be adultery in His eyes, how much more serious would it be if it’s beyond a lustful thought, and it’s with sexually someone you have no covenant with?
I don’t doubt there could have been varying opinions on this subject throughout Church history, but that was the case with other issues that the Church had to formally define as dogma. Even saints haven’t always been on the side of the consensus view that ultimately prevailed.
I’ll have to look at these references in your site, it is rather extensive, but thanks for sharing it Very quickly though there many Church fathers and Doctors, who appear to teach the Catholic view, but to keep it brief I’ll just refer to the council of Elvira that you alluded to in your link:
" …I would like to clarify the current academic assessment of Canon 36 of the Synod of Rome (826), add both the Council of Elvira (c. 300) and the Council of Adge (506) as in favor of divorce and remarriage…"
I’ve read Elvira differently and it sounds a lot more like the Catholic teaching, notice in the first sentence that the husband is the one presumed to have already committed adultery… Even if the wife was not the cause of the husbands adultery. She commits adultery if she remarries and he is still living.
Council of Elvira:
Likewise, a woman of the faith [i.e., a baptized person] who has left an adulterous husband of the faith and marries another, her marrying in this manner is prohibited. If she has so married, she may not at any more receive communion — unless he that she has left has since departed from this world (canon 9 [A.D. 324]).
If she whom a catechumen [an unbaptized person studying the faith] has left shall have married a husband, she is able to be admitted to the fountain of baptism. This shall also be observed in the instance where it is the woman who is the catechumen. But if a woman of the faithful is taken in marriage by a man who left an innocent wife, and if she knew that he had a wife whom he had left without cause, it is determined that Communion is not to be given to her even at death (canon 10).