I'm on board for fade outs on playback stop if we don't already have this but after reading a little more of your issues something jumped out at me.
You said the problem wasn't there with 1 track but was there with more tracks. That does sound suspiciously like an issue beyond track fade outs, as you said it is exacerbated by multiple tracks. basmeijer, I think you should create a tiny test project where you can demonstrate this behavior and use a loopback and another DAW to record the monitoring output of your interface. REAPER's pretty lightweight and you mentioned it before, so that might work for this. Then if you file a support ticket with the test project, and maybe a screencap with LICE Cap or something like it to show what the circumstances of the spikes are, then that would give support more to go off.