Hi! I'd like to answer your questions and share some feedback to help take this to a more refined state. I'll answer each question in the order you've asked them:
- The composition and vibe are delivered nicely within your track. Violin and keys being slightly spread apart in panning gives a nice touch of clarity and sparks interest; having both instrument tracks right dead in the center isn't so fun, especially if they initially (before any processing) are mono, right?
- I had no experience with game engines to give a good answer, but having stems for whatever purposes (remastering to fit better in game's style, remixing, programming the triggers for audio cues, ...) is always a good thing. I think it's always best to include it - maybe some would be happy to simply use the soundtrack as is while other may want to edit the audio to suit their tastes and project's style.
- Maybe the violin has a tiny bit too much of harsh frequences. A simple subtracting bell would be enough, just make a ~1-2 dB cut somewhere in the upper mids and it's alright. What is a buzzkill here though is those bass drums and no proper bass. The former seems to be overly loud, but maybe it's because it has no other bass-occupying instruments in the mix. I could hear some low freqs in the track, but the keys don't have low notes programmed (or performed), so there's a hole that creates the unwanted contrast that also accentuates those bass drum hits. I'm sure that there are free VST instruments or Kontakt libraries that would help you fix that last issue.
Overall, the work is solid, I liked it! Considering it's your first project of this kind, you've done well.