Re: Am I the one who posted the "clever trick" link?
Maybe. Sorry I can't remember now

Re: Identities as commonly understood don't work in exchange.
Yep, I think this was the conclusion when this was discussed in detail, here.
Re: for my O365 account, access email through imap (where identities work fine) and calendar / contacts through an exchange account
This works, yes it's not that great, but...
With IMAP, AquaMail can send mail through an identity's own SMTP server, so Exchange *server* isn't involved in sending at all.
Suppose it should be possible to do this for Exchange accounts in AquaMail too -- require "own sending server" when adding an identity, of SMTP type.
I believe this was discussed too at some point, but again I'm not sure if this is a truly universal solution (maybe there are users who prefer doing it the MS way, with a distribution list).