Partly related to what you are talking about: before the pandemic 3 times I had problems to return to Europe (I'm European) from the States because according to the check in staff I needed to provide proof that I would be leaving the country I was traveling to (Norway) in less than 3 months. These were all Delta personnel, clearly unaware of Schengen and entry rules for Europeans. Twice I was lucky enough to have another business trip programmed and be able to show tickets to prove I would not "overstay", but another time i had to book a quick Oslo-London Ryanair flight for 20-something euros to show it as a proof that I wasn't going to overstay Norway. These were all the return leg of my EU-US-EU usual business trips, and not even explaining that I am not "going to" Europe but "coming back" and showing my itinerary would help.

What I mean with all this is that sometimes airport and airline personnel might be wrong about the rules that apply at destination, and as a passenger you are always exposed to that risk. Good luck @b12e with not running into staff that doesn't have the latest info, because it does happen....