I am curious. Does someone who has re-upped next season with their 5 credits still get that team if they haven't even logged in this season?

If someone doesn't log in for the last quarter of the season or so (don't recall offhand exactly how long) they don't get to keep their team even if they have renewed.

what if they log in but not into that team?

It's looking at the general activity, so if someone does abandon a team but is still playing in other leagues that is a loophole in this protection.