What would Sidney and Clifford do if there was no heart attack?
Sidney and Clifford's plan basically required Myra to be scared into having a fatal heart attack. I wonder what their strategy would have been if Myra did not die, assuming they thought through some sort of backup plan? Their options for plan B:
1. Kill Myra outright with any means handy - in which case they don't have the advantage of having it look like death by natural causes, and they'd have a lot of explaining to do as to why she's disappeared (assuming they could conceal the body successfully).
2. Play dumb with Myra and say that it was just an over the top rehearsal for one of Sidney's plays, or try to laugh it off as a bad gag gone wrong.