What determines which application acts to a bluetooth AVRCP command?
media-button-router, already mentioned in one of your linked questions, tries to figure out what app is currently playing music and to dispatch the AVRCP events to it. In addition, it lists all apps installed, that declare to listen to AVRCP events.

What makes AVRCP complicated: Some apps, like Sony's Music Unlimited, seem not to declare android.intent.action.MEDIA_BUTTON in their manifest. For this (or maybe some other) reason they can't be controlled via media-button-router. I know that e.g. Music Unlimited can be controlled via AVRCP since I read it in a user comment and also I was able to get it running once but unfortunately it's not reproducable. So in those cases it seems that we have to wait for an app's update with a clean implementation of AVRCP event handling.

According to media-button-router's wiki "Media Button Intent handling is done through ordered broadcasts, and each app sets it's own priority for receiving." So maybe it could be possible to write an app that displays theese priorities and, in effect, would allow you to determine what application will start. However, I don't know about such an app.