I've got a full carbon frameset which has quite a long steerer tube and a very upright position. I'd like to slam the stem but dont want to commit to trimming the steerer tube in case I need to change my position or if I ever sell the frame.

The frame in question is a Planet X model and they don't have any guidance regarding this, but Cannondale say with their forks you should have a single spacer above the stem so the clamping forces of the stem are distributed around the fork and expander bung evenly, whereas Specialized explicitly state that no spacers should be used above the stem.

1. How many spacers is it safe to have on top of the stem?
2. Should the full carbon steerer be supported on the inside exactly where the stem clamps with an expander plug?

The answer is dependent on the sort of compression plug you use. If it's the type that has a top lip whose outside is flush with the steerer tube, then no spacers are typically required above the stem. If it's the type of plug that sits completely inside the steerer, they usually recommend 10mm spacer above the stem IIRC.

Generally the only concern about lot's of spacers ABOVE the stem is the risk of getting impaled on the extension if you crash. (or banging your chin on it if you get into an aero tuck.

There is an advantage to having the expander extend below the stem, but IMO this is of more concern to the belt and suspenders folks. The reality is that a steerer is far more likely to fail at the headset than under the stem. In any case, most expanders don't extend below the stem anyway. If you're really concerned about steerer failure, find a wooden dowel rod if the right diameter, and drop it into the steerer so it transitions the distance from below the headset up to the stem, or as close as possible.