4) The 5 boxes are a minimum, the more you’ll select the better. It also helps to place them evenly spread across your image. If you put 20 boxes in one corner and only 4 in the other corner, that corner could have less correction as you would like. So each box has a weight in the results.
The LP correction for calibration is indeed way to cumbersome. It takes forever with a lot of frames.
In the RFC list, is the proposal to make this a batch tool for calibration. For the first frame you select the boxes and then the tool would apply those boxes to all the frames that you are calibrating. It should work really well after you have registered those frames first. The registration will ensure that good areas are used automatically in the gradient/LP correction of all frames ;-) so this feauture will come soon. It will be really helpfull. (So your proposal in already on the RFC list)
The Local Normalization Correction during stacking will also work pretty well, it causes all layers in the stack to become evenly illuminated over the entire field of view, giving you better data normalization and much less stack artefacts. But the LNC won’t ensure a completely flat stack, whereas this proposed LP calibration batch tool would ensure this. So it will be very usefull, no doubt ;-)