Thanks, FB. Great list to check. Here's where I'm at:

1 - check, everything is (reasonably) loose. With the carbon, seems to be a little more friction than with an AL steerer, but I've tried with some grease to make sure everything can slide freely.
2 - check, have at least 3 mm space at top, maybe more like 5 mm.
3 - check, added friction compound, plug seems nice & tight & can't perceive any slippage.
4 - hmmm...interesting thought. I'll have to take a closer look at this suggestion. I'll try a longer bolt.
5 - check, the spacers don't seem to be an issue, as I've switched them out for clean, AL ones for testing purposes. Though some of the carbon spacers I have don't seem to be completely flat.

The loose stack problem makes me wonder as well, wish the problem was a little more obvious... My perception is that there is minimal play at the crown when I press the fork upwards, but it's really hard to tell.