Hi -
I'm glad you found my suggestion useful.
WRT using the levels adjustment sliders to adjust the endpoints inwards, this will definitely prevent PS's pixelate/halftone filter from producing either total blank areas or merged dots - I just double checked that it worked with that filter. Just be sure you are moving the correct sliders. In a levels adjustment layer, you need to be moving the lower sliders (ie, the two "output levels") inwards, NOT the upper three sliders.
I have been using VanDerLee's plugin for ages, and consider it the best bang for the buck compared to RIP software. My only gripe with it is that, if I'm not mistaken, he hasn't updated it in a while, so it only is compatible with 32 bit versions of PS, not 64 bit versions. That's not a big deal if your files aren't huge, as even the latest versions of PS CC allow one to manually install the 32 bit version of PS alongside the 64 bit version with no problem whatsoever (...that's what I do). Do check, however, perhaps he has upgraded it since I last looked. BTW, it does work on 16 bpc files, not just 8 bpc files.
To answer your last question, yes, VanDerLee's filter is only B&W, so, you have to do the separations manually and recombine the 4 colors. OTOH, the ability of VanDerLee's filter to save groups of settings makes it a lot easier to do this precisely.
Although it certainly is not the same as a halftone filter, from what you have said, I suspect you might also like "Engraver":
http://www.alphaplugins.com/products/products.php?menu=get_prod_id&prod_id=2
HTH,
Tom M
