I'm too waiting for some good weather to test on field. As for guiding, right now I will only do unguided exposure, this is the reason for my interest in PEC. Yes, I saw several posts as well about weird behaviors with PEC+guide, but there are also some guys who had no issue, and others for which it worked for some time and then stopped. I don't think it's a matter of double correction. Guiding corrects the error after having seen it, so with a perfect PEC on, periodic error cannot even be seen by the guider (in reality some error will still spill). The issue is probably due to the fact the at the same time both PEC and guider would like to apply a correction (non necessarily of the same sign and amplitude) and this is not well managed by the firmware. Combining the two corrections in the driver would work (you would start applying only the PEC, and then add the reactive guide for the residual error), but, as you said, requires the driver to know where the worm is, which can be inconvenient. You should always park and unpark, but I think that also any manual slew with the mount not monitored by the driver has to be avoided.