Btw, I'm wondering can a slot provider run the competitions we are talking about, or only an online casino can do it?
They can if they wish, why not?
Many providers run competitions globally (players can participate in any casinos where the provider is available), so endorphina can do it as well in this forum.
They can simply create a competition such as what you mentioned (multiplier contest) and participants should play their games no matter where players want to play the game.
I have a feeling that Pragmatic, Booming, Playson, and Spinomenal are running competitions all the time... As Panjul07 said, it doesn't matter in which casino you play if the casino participates in the promotion of the provider, it's the provider who chooses slots/live games. And yes it's global, so competition is crazy hard! Depending on the competition min bet can be $0.5 - $1... I tried to get some points/tickets in some competitions before but without some big success. Winning some higher place with the prize requires a lot of playing, to not mention a nice bankroll.