Please re-read the relevant part of my last post.
Can I reiterate the point that RF channel BW is almost irrelevant in a digital world. Obviously the wider it is, the more bits per second you can carry for a given error rate. Hence for a given compression system and modulation method you can carry more programmes in a wider RF channel. If you use a more modern compression system such as MPEG4 you can carry more channels or get higher quality or any tradeoff you like between them. Likewise if you use a more efficient channel coding scheme. I'm not sure how 8VSB stacks up against COFDM but I know it's not readily feasible to have a more efficient scheme than COFDM. Again I'll re-itereate that the only purpose of channel coding is to get all the bits from the TX to RX with acceptably low error rate and in an acceptable amount of RF bandwidth.
Multiplexing is simply a means of conveniently utilising the carrying capacity of the RF channel. A single 8MHz channel (or 6MHz channel for that matter) has ample capacity to carry a number of MPEG2 compressed SD programmes or a smaller number of MPEG4 compressed HD programmes. The exact number depends on how hard you are willing to compress the video. A single programme carried in that much RF BW would be a gross waste of spectrum. I don't know if multiplexing is used with 8VSB. If in the US you are using 6MHz of RF to convey a single programme then that's gross waste of RF spectrum. Maybe you've got more of it over there than we have in Europe
I know the real reason why multiplexing is unpopular in the US. It's a socaialist plot to force the sharing of transmitters between competing TV stations.
Chroma BW is a decision made at the start of the compression process. Since the source material will usually be 4:2:2 (for SD) or multiples thereof for HD horizontal chroma res will be half of Y res. Vertical chroma res will be the same as Y res. Which is why it's usual to reduce vertical C res to give a 4:2:0 picture at the input to the compressor. Or multiples thereof for HD.
It is possible to use full BW chroma, the SDI specs and MPEG specs have options for 4:4:4 and multiples. While this may be useful in film production it has no place in transmission.
The choice of half res chroma was made back in the early days of digital experiments. I think it was settled as 4:2:2 around 1990. Can't be bothered to dig out the papers. A lot of work was done with chroma a third of luma BW but this was considered inadequate for downsteam processing and chroma key. It's proven to be a good and practical choice though a few users may have a need for full BW chroma.