Okay,
Here's what actually happened in case any of you have the misfortune to fly UIA on a KL ticket 😁

Lounge access: Yes
Fast track security: Yes
Priority check-in: Yes, but only when said I was "business class" (the dragons guarding the business class check-in desks did not seem to know about FB plat status) but when I got to the desk it was no problem once I flashed the card.
Check in-bag: Yes, I could check one bag for free
Seat selection: Regular eco only, exit row was full but it might be possible to get it for free if the other seats are occupied.

Now, here comes the surprise. I was charged 15 euros as a boarding-pass issuing fee because I had not checked in online. It turns out UIA charges a Ryanair-style fee if you fail to check in online.

I wrote to KLM about it afterwards and was refunded the 15 euros as nothing is written on the KLM partner page about having to check in online with UIA. Actually, the KLM partner page said it was possible to check in via the KLM website but that wasn't possible when I tried. So the info was incomplete/out of date. KLM apologized and promised to update it.

All told, if I had the choice again I would actively avoid UIA as it really is a low cost airline. The 3 hour flight to AMS felt longer than usual because of the lack of free meal service.