1- No point in messing about with the "preferred tuner" setting: just leave it to the default (= auto) and the tuner with the least number of satellites will be preferred by the system.
2- Not quite sure, but I think basilyoung is pointing in the direction of a possible tuner problem. Tuners can deliver up to 400mA to the coax (= to the motor + LNB in your case); especially on start moving currents may be higher for a moment. Normally the tuner is protected for that, but of course such a protection could fail. So indeed I would swap feeds, and see if tuner A on the fixed dish works fine. And if not, I wouldn't dare connecting the motor to tuner B.......
3- Not sure what the setting "use current measurement" on the ET's does exactly, but maybe it's worthwhile turning that to off.