Used the fathersday to start from scratch again and this time with another RPi i still had flying around. While at first I had the same issue with the bluetooth streaming it is now working.
Seems as if the bluetooth streaming uses always the device #1 even if the on board sound is disabled and the AMP2 being at device #0 position.
However, my AMP2 somehow did not play via squeezlite first when on-board sound is not disabled and the card is #1. So I disabled on-board sound change hw:1 to hw:0 with the audioplayer and it played well. Bluetooth however not at this time. I the enabled on-board sound again and magically both, bluetooth and audioplayer worked well.
Strange but at the moment it is solved for now.