To answer your first question, panning of individual tracks is not allowed in LP-16. What you hear from the stereo head phone output is only a mono downmix (on both L and R) of all the channels. As Laszlo also said earlier, the line outs themselves are all mono outputs. Hence the option is greyed out in the uTool. Remember that the Mixer inside the uTool, is a Direct Out mixer. Changing the levels of a track with this mixer also affects the actual output on the line outs.
About the second question, a multitrack song, should contain only mono wave files. You cannot place any stereo wave files inside a multitrack song. So you cannot place 8 stereo wave files inside a multitrack.
Apart from the multitrack mode, the LP-16 has another mode (stereo mode) where it can playback ONE stereo song at a time. This single stereo song can be played back on any 2 consecutive channels on the line out. The Left will go out on the first channel and the Right will go out on the second channel. The output channels in stereo mode can be changed in the settings menu.
I hope this clears up some of the confusion.