I'm really thinking that a DV->Mpeg AviSynth filter, like Reinterpolate411 is worth a try, for that problem.
Mpeg samples color between two lines, like this:
--
--
But NTSC DV samples color along one line, like this:
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So an unfiltered conversion would look like horizontal lines (line segments), of color error - often blocky-ed across two lines (I don't know exactly what it'd look like, but those screencaps fit the theory).
It also looks like denoiser is getting confused by minor chroma noise, because of the DV. (Not sure if that's curable).
Don't know why it'd only show up after IVTC, though. But this doesn't look like IVTC errors, to me.
OTOH, maybe those line segments are screwing up the IVTC detection, and it's blending/averaging/interpolating every single field. Moth3r recommended manual settings, but I don't know if you can set it separately that for each source. Maybe you can tell decomb not to look at chroma when detecting telecine patterns.
Something they didn't mention: 24p is also kinda nessecary for editing together different telecined captures. The different telecines wouldn't be synced together, so without IVTC, you'd all kinds of stray-field problems showing up at a lot of edit points.
(Oh, and sorry, I gave you the wrong link, earlier. It mentioned some DV-Mpeg filters, but the main subject was about encoders. Sorry to give you the wrong impression. I'd promise to look up better threads, but I probably wouldn't, so I won't promise - I rarely get much forum stuff done lately).