Erica said:- "So I don't know if the issue is that Sir Thomas himself objects to private theatricals, but that for the Bertrams to be engaging in such while their father is gone is what is objectionable."

From what we know by ch.13 I think it's pretty clear that Sir Thomas would disapprove of private theatricals. He wishes to see all who are connected with him in respectable situations (ch.1)n and hopes that his daughters make "respectable" marriages. He values Edmund's character and thinks that" his strong good sense and uprightness of mind, bid most fairly for utility, honour, and happiness to himself and all his connexions". (ch.2). The Miss Bertrams appear to their father to be "all that could satisfy his anxiety", and he trusts that their marriages will bring the family "respectable alliances",(ch2), but they have always felt constrained in his presence and he has "never seemed the friend of their pleasures", (ch.3).

So Sir Thomas is much concerned about honour and 'respectability', and by implication cares what other people think of his family. He approves of the restrained behaviour he observes in his daughters, and of Edmund's seriousness. Not a man who would be pleased by private theatricals at any time I believe.

I think Edmund is especially alarmed by, and Sir Thomas would certainly deplore the sort of publicity which would bring "fame, ..... the long paragraph (in the press) in praise of the private theatricals, .... and immortalise the whole party for at least a twelvemonth!"(ch13).

As to acting while sir Thomas is away, and perhaps undertaking the hazardous journey home from Antigua, Edmund's concern seems well justified to me. It means that the theatricals do not have the have the evident parental sanction which Sir Thomas' presence would give. And what if Sir Thomas should die of a fever or be shipwrecked. The press and the gossips would have a field-day! Those rackety Bertrams! .... shamelessly cavorting while their father is in deadly peril!! ..... quite shocking!!! ...etc, etc.