Google Play Console only works with your primary account.

Over the years they have definitely fixed those one by one, but I’m not sure if they’ve fixed all of them. I believe the last problematic one I encountered was some ads-related developer console a year or two ago.

They fixed the big ones but there are still a bunch of edge cases where you have to log out, like one account is from US and one is from Canada.

No. I use this feature daily, and have to login to my "primary" Google ID for my Drive/Calendar, then I "Add Account" to enable my secondary/tertiary ID's. It has to be in that order.

Good question, though the account isolation to the extent I've tested it has been surprisingly good.

I still don't fully trust it, but that's a different matter.