When I went to Eurovision in Malmö we went for one of the semis and we're back home on the plane for Friday. I then had a Eurovision party and was on here.
So yeah don't assume that everyone in the arena for a semi will attend a final. They are also likely to be lightning rods - they will have lots of friends who will not be at the gigs but they will be sharing on social media.
I've found our staging the last few years not arena friendly. Olly Alexander's box would have had lots of issues with lots of people in the area not being able to see him for much of the performance. It sounds like it was similar for Remember Monday. (We won't talk about the utterly pointless trumpets from another previous non Sam Ryder year)
There was one entry in Malmö that had similar issues which was really annoying. I think it was Kaleen We will rave (Austria). I really liked the song but there was this stage thing which meant you could see her for about half the song. It really takes away from the performance. Whereas good acts can have great visuals and costumes but don't need the props. (Cha cha cha is a good example - he had props for the performance when competing but when he has subsequently performed it as a returning artist he did and it didn't take away. If anything it was better. Europapa had great visuals but the actual staging was minimal and all about the costume and dance. Also see Baby Lasagna. Tbh Greece this year was a bit of a mess and had a pointless scooter as it was all over the place and wasn't relatively clean so to speak - it was trying too hard to be everything).
Bulgarias use of staging and chairs was great. It didn't take away. On the unrehearsed performance at the end she and the dancers briefly found a chair in the green room to do their routine before going onto the main stage. That performance should have won anyone who didn't think she was a deserving winner over. It was all so professional whilst they were all clearly bemused and winging it.