'Which game' should also be accepted.
That would be "kumpaa peliä..."
Only if there are two games and you ask which one of the two.
In a question like this, could the partitive peliä also be translated as plural?
like Kristian said, no. plural would be "pelejä"
Is "pelia" only used for videogames or also for regular games and sports? Would I use the verb "pelata" if I wanted to say: "I like to play football"?
I cannot answer your question, but it's "peliä" not "pelia" with the latter one disrespecting Finnish vowel harmony. The usage of "a - ä, o - ö, u - y" is essential when learning Finnish.
Yes, but the diaeresis or umlaut is not available on all keyboards. I suspect that anyone who has gotten this far in a Finnish course will be aware of the difference between ä and a.