Crabtr

> on top of songs disappearing from my playlists

It might have been the case that the song didn't disappear, but is instead hidden because it's unavailable.

At the bottom of the preferences window there's a section called "display options." In there enable "show unavailable songs in playlists."

That whine's almost 100% coming from the CPU as it's a common issue with Intel CPUs these days. It's the weirdest thing ever, but my Hackintosh makes it as well (i7-6700k) so you're not crazy. It also very notably comes and goes independent of workload and mouse polling rates, but no matter the case I'd still return/replace the Mini just because it's an option.