I am glad that this issued is posted. Our first encounter with infiltrators was in Edinburgh, Scotland. There for 3 weeks visiting and each week in July saw trouble in SSPX. (July 2019). Then spoke with people from Malaysia and they too experienced what we did. They said the word "infiltrators", then I agreed, yes!
This should not be surprising, but the parishes need to keep note of being watchful, aware, and give them no attention.
What we noticed was women demanding not to wear head coverings, wearing slacks. Then Priest having a homily on Christ present in both species. Then after distributing Eucharist, 2 people come to the communion rail, with white gloves and both receive wine. (We know that a crumb is all needed to have Christ). The next Sunday, Benediction after Mass. After Eucharist, half the people left and never came back! Very sad, but that was SSPX in Scotland. We went as observers and we kept Our Sunday Holy.
I know of a Traditional Church with independent priest being watched by New Order to report back to bishop of dioceses. That priest knew of it. 1986.
I do think it should be an issue to speak of and how to handle it. Ushers are very important to watch and to maintain dress codes and such. Once they start, if you don't stop, they will continue. You can start off "friendly" and how is that defined? But you must nip things in a bud. We know that Christ is Truly Present, then be sure to have greeters, and that they do their job.