Derotation is an integral part of our software, permitting long exposures of the targets (there is no mechanical derotator built into our products).
As for the epoch, it's J2000, and then the software does the job.
The maximum angle is 85°, however, we recommend staying under 80°. Please don't observe near the Zenith, 20° would be the lowest. The azimuth can move quite quickly, requiring quite a bit of rotation. Compounding this is the fact that the closer you are to the Zenith, the less leverage you have to rotate the scope. Near the horizon, the tube is nearly horizontal and you have the full length of the tube as a lever. Near the Zenith, since the tube is nearly vertical, you only have the diameter of the tube as a lever, much shorter, the effort is greater and fine control is reduced.