I pretty much agree with everything in this thread. Robbin's sympathetic take makes sense, and so does yours.

You're right. Sir Thomas is relying on Edmund.
ch 3
but, in Mrs. Norris’s watchful attention, and in Edmund’s judgment, he had sufficient confidence to make him go without fears for their (daughters') conduct.

I do think Edmund is in a difficult position because of Tom's return. Tom is more the head of household with Sir Thomas away, and Edmund has no hope of controlling him. Maria and Julia are willfully happy to ignore Edmund's objections to the acting. For him to give in himself is weak considering his vehement objection to the family acting and then Lovers' Vows in particular.

As you said Fanny's suspicion about Henry's attentions to Maria should have given Edmund pause. If he hadn't noticed anything before, you would think he would at least pay more attention to what Crawford and Maria were up to after that. But he didn't, so I have to agree that he pretty much ignored Fanny's concerns. Badly done. Even if he was unable to control or influence his sisters, I think a warning from him to Henry would have stopped him in tracks.

If marriage is a take-in as Mary says, then a wedding must be on the way, because Edmund is well and truly taken in.