The comb effect and black of your video, they are not related, probably.
I never shot in 30P but I do a ton of 24P work for DVDs.
If the footage was shot in 30P, captured 30P and you want a 30P DVD the timeline shot be Progressive. Switch back and forth is never a good thing. (29.97 is the correct frame timing)
If record and capture were 30P set your timeline to progressive, if you have to render all the footage something is wrong.
Export to compressor from FCP
Make a new CBR, yes CBR mpeg 2 present, with 29.97 and dominance set to Progressive. Make it as high as you can (up to 7Mbps) and then encode the audio as an ac3. This should help the black problem. Blacks is a hard thing to compress, and VBR does a crappy job of each.
Keep field order (or the lack of one) constant throughout the entire process. This should fix your combing problem.