*I rendered out a clip of the chromakey (using no compression) without using lower-field-first. It looks fine.”
ok, here’s what’s up, when you render a betacam out with fields off or a psd, you’re rendering out a poor man’s deinterlace from AE’s built in deinterlacer. That’s why when you reimport it back into AE and try interpreting it with fields on, you get blur. Of coarse you do, you’re trying to deinterlace deinterlaced footage!
The quicktime codec might be giving you trouble. Try a different codec and render that out fields on. Chances are, you will then successfully import it back in and it won’t be blurry. Whenever I have a problem, I render out 10 seconds of a 4:4:4: RGB uncompressed avi. That fixes 99% of problems.