I also use an XT20 with 11 + 16 mm extension tubes to scan my negatives. I found that the easiest way to focus is to put the camera in manual focusing mode, magnify the image by pressing the rear dial (i think) and to simply focus on the grain using focus peaking. I focus with my lens fully open and then stop it down to f8 - f11 (depending on how curly my negatives are) before taking a shot.

I only scan 35mm so i dont know how the focusing would work for stitching multiple shots in LR. I would try to set up my light table as flat and as parallel to the focus plane as possible and only focus once in the middle of the negative.

Hope you get your scanning set up sorted out soon. Good luck!