i literally just needed to log in to someone's account once, for an hour, to substitute for them in one call.
I get your frustration, but that's a fucking awful choice for a whole bunch of reasons, not only because of Microsoft's bullshit. You really buried the lede.
Don't share accounts people!
Have them forward you the meeting invite or reach out to the organizer to call you instead.
And if you fucking insist on logging into someone else's account again, use Private Browsing mode and the web client of Teams to keep it from touching the rest of the machine. It works fine for audio and video calls.
At any place with a half decent security policy you'd be looking at disciplinary action. At certain workplaces this would be an immediate firing.