1: That would be Närcon, that's for sure
Especially in Sweden.
However, there's also Stockholms internationella Seriefestival which takes place in April/May every year at Kulturhuset (they'll change location next year due to renovations.
I do want to say, the market is THIRSTY for manga right now, so any convention would be a hit I belive.
There's also Örebro Seriefestival (In November), Malmö Seriefest (March), NärCon Höst, Sommar, Vinter and Comic Con.
2: NärCon has gotten ALOT of complaints regarding their artist alley, so has Comic Con as well. NärCon Sommar has the most space for the artist alley, but I feel like the convention itself doesn't really want it, because, as what they've said themselves "They don't earn any money on it".
3: I've been to ALOT of places, Thought Bubble in Leeds has been my first (and so far, the only) one that has been outside of Sweden.
I sell much, much more on Swedish conventions, mainly because I have a fanbase here and not so much overseas.
And I would say: It depends a lot on how you're able to sell your stuff. Some have their art sell for themselves, while others need to push that extra mile to be able to sell.
So, saying how much one sells is kinda personal as it depends quite a lot on merch, the availability, how you're able to sell it and so forth and so on.
4: Närcon has one of the highest rates, from 400-800 SEK. I don't know about Comic Con, but I was considering going for their first year but didn't feel like paying 3000 SEK on one smol table.
I think they lowered the price tag on it, but I'm not so sure.
Thought Bubble has about 1200 SEK/table or something but it's also REALLY big.
Örebro Seriefestival is free atm, Stockholm (THE biggest comic fair there is in Sweden) costs about 500SEK/table. They could be raising the prices due to selling out in less than 5 (!!!) minutes.
5: There's a STRONG market for original work, as long as it's eye catching.
It also depends a bit on your work, are you selling comics and/or posters + merch or just the merch part?
Stickers almost always sell well if set for a low price.