Can this not be translated as "My grandfather is currently playing the violin"?
That should work, as «сейчас» refers to something at the present time.
I had put "My grandpa is playing the violin right now"--I think that's also an acceptable answer
Report it. "Right now" is "прямо сейчас", though.
Guys, what is the true translation of "Granddad" into Russian?
Is this in the Prepositional, or the Dative (when you play an instrument)? What about games, board games, sports?
Instruments are put into prepositional (на гитаре, на скрипке), games/sports are accusative (в футбол, в карты).
How on earth can "My grandfather is now playing the guitar" be incorrect but "My grandfather is playing the guitar now" is correct? They mean exactly the same thing.
Both are incorrect, because it's a violin, not a guitar.