I do the same thing as OP - I have multiple separate containers, each has its own set of cookies, and its own Google account that it is logged into. I just change my window when I want to use a different account.

From what I understand, you're asking how you can just "temporarily" log into an account — can't you just add that account as a new user on Chrome?

If not, you can always open an incognito window or a guest window, do your business, and close it. I do that often when I'm logging into an account that I don't want to persist.

Then make each chrome account a different person tab and you won't accidentally pollute or cross pollinate your state. If you don't want using Chrome for Gmail (excellent js Gui) to pollute search and other work, might need to see if this kind of segmented behaviour works. Otherwise its two browser types: Chrome for one tab of mail and ephemeral tabs for clicked out of mail, and ff for everything else?