Whilst digging around for contextual info which might shed light on Edmund's objections to the play, I came across the following passages in "The Gentleman's Daughter. Women's Lives in Georgian England" by Amanda Vickery, 1998.

The first relates to Edmund's statement (ch.13) that the play "would be imprudent, I think, with regard to Maria, whose situation is a very delicate one, considering everything, extremely delicate."

Vickery states (p.53) that it was in a genteel girl's interest for marriage to follow speedily after betrothal. "Engagements which collapsed at the settlement stage tainted a woman's reputation, so publicity in the nervous months between the promise and the wedding was a mixed blessing for elite brides." She gives the following quote from 'History of Louisa Mildmay' by Hugh Kelly, 1767:
"...of all the stages in a woman's life ... none is more dangerous as the period between her acknowledgement of a passion for a man, and the day set apart for her nuptials."

This makes Maria's entanglement with Henry look doubly dangerous. It also highlights the stupidity of Mrs Norris when she so busily spread the news of the engagement.

Regarding private theatricals, Vickery says (p.253-4) that these were at their height among the elite in the period 1770-1810, and attracted intense public interest. She comments on how women acting was at odds with the ideal of demure dignity and could undermine their reputation. Vickery continues as follows, citing "Theatres of War" by Gillian Russell:-

"Yet perhaps the very shattering of the ideal of modest female reserve was politically calculated in the first place. ...Russell reminds us that 'many leading Whig families were identified as enthusiastic thespians; in this respect amateur acting can be regarded as part of a repertoire of behaviour - including libertine flamboyance, female exhibitionism, and the cultivation of public celebrity - that was used to define the Prince of Wales's party in defiance of the "Farmer George" probity associated with the King'. Russell argues that private theatricals were not only an important expression of country-house paternalism, but also a mechanism for the consolidation and enactment of social alliances among the political elite, an arena in which women glittered centre stage in conspicuous contrast to their negligible role in more traditional fora. ...... Of course, the modest family theatricals mounted by the gentry lacked the political punch and the titillating excitement of these aristocratic extravaganzas. Minimal publicity, a small audience, a suitable play and senior family members en costume put the stamp of innocent diversion on family play-acting. Still, an unsated interest in more outrageous productions remained widespread amongst the genteel."