Sorry for the problems you have been facing.
— which should output a discrete headphone mix for the drummer via headphones and a mix of the other tracks via the stereo op.
There is only one mix down in the LP-16 which comes on the headphones out on the front of the device. The stereo jack at the back is a “line in” and not an output. So the mixer you see on the uTool application is for the headphones.
The bug regarding the uTool2 is already noted and a fix will be available as soon as possible. In the meantime you could try to use the first version of uTool. Unfortunately stereo files are not support in the first version and you would have to split them into mono files before you can create the multi-track songs.