MacOS – Invitations sent to Gmail do not appear in Calendar while invitations sent to Office 365 do

Under Accounts in Preferences, there's a field there called "Email Address:". You can put a comma-separated list of addresses in there, like this:

Now your "Compose" window will have a "From" popup menu that lets you choose your sending address:

AFAIK, this works for pretty much any mail account type, as long as your mail provider allows you to muck around with the From: header.

I am fairly "late to the game", but since no answer has been accepted yet, my input might still be useful:

• the "meeting highjacking" you describe in your question is fairly well known with Exchange and the iOS calendar app; in fact it can be even worse where simply accepting an invite on your phone may make you the "owner" of the meeting; this seems to happen more often the larger the number of meeting attendees gets. This issue is so severe that in my company we discourage people from accepting meeting invites from their phones.
• as for the "butchered description" you describe in your answer: this, too is a well known issue between Exchange and the iOS calendar. iOS 8.3 supposedly fixed it, but since those cut off notes do not happen all the time, we do not have enough data yet at work to say for sure.