I have a doubt about the question of communicatio in sacris. That's a "continuist" objection to the traditionalist's critic of ecuмenism of the Vatican II. Did it exist before the Council? Was it really licit and necessary? How to answer this kind of objection?
Not exactly sure what you're asking but I'll take a stab at it. Below is from the 1917 Code of Canon Law (PDF attached below)
Canon 1258
§ 1. It is not licit for the faithful by any manner to assist actively or to have a part in the sacred [rites] of non-Catholics.
§ 2. Passive or merely material presence can be tolerated for the sake of honor or civil office, for grave reason approved by the Bishop in case of doubt, at the funerals, weddings, and similar solemnities of non-Catholics, provided danger of perversion and scandal is absent.
Commentary on that canon says:
VI. Suspicion of Heresy Falls on
1. persons who knowingly and of their own accord help to propagate heresy in any manner, or who take an active
part in the Divine worship of non-Catholics, as forbidden by Canon 1258...