Yes, and yes. I've used USB Audio Player Pro (UAPP) for years, it takes full advantage of whatever DAC you're using and offers great control. It doesn't support Spotify, though. I think it cost $7 or $8.
There are other apps, too. Hiby, Onkyo, and FiiO come to mind. I can't remember why I preferred UAPP to Hiby and Onkyo, but I did. I think UAPP sounded better than Onkyo, but I don't really remember. I tried the FiiO app several months ago and thought it was a clunky, unintuitive mess. Maybe things have changed.
Yeah, I've used multiple music player apps with my FIIO M6. APK files are very easy to get, FIIO actually have a bunch of good ones for you to download. No problems. I get the same performance with them as I do with the preinstalled FIIO player.... which to be fair is the one I use anyway. Tho I do use VLC sometimes.