I think the sofa table painted black could work. I would be a little worried about the brass colored edging. Wonder if it could be painted over? Could look nice if it could be made to match the silver of the mirror. One possibility with the cabinet is remove the doors as you suggested, and then possibly add dark colored glass doors to make it look more purposeful as an entertainment center. In general, the trouble I have with this piece in your living room is that you have a very clean cut look - and really it's very striking. The mantel is a very hard edge on the top, while this hutch has a rounded edge. By taking the doors off, it could make the piece fit in better. If you were more into eclectic or cottage style, then the hutch would be an obvious fit. If you do use the hutch, I would recommend using sleeker brushed nickel drawer handles that don't dangle & clang. Is the hutch solid/real wood? If so, I would play first with the sofa table because it is a more dated style that really wouldn't be hurt by paint/updating. If the hutch is solid wood, then there would be more value in keeping the wood visible, imo. Overall, I have to say that I really, really like your living room. It is simple, yet very eye catching in a sophisticated subtle way.

Plenty of them at Hooker Furniture, they have one of the larger selections available. You can see the styles here: https://www.hookerfurniture.com/itembrowser.aspx?action=attributes&Department=Home%20Entertainment&Type=Entertainment%20Centers&SubType=TV%20Stands#top

80" could be correct for the TV and you can purchase them from Wayfair, Overstock, IKEA etc. However, depending on what you want the purpose of the wall to be 80" may only be part of your solution so don't rush, ...think this through and come back for help if needed.

All of those shelves are too high and visually detached from the rest of your decor. I would hang shelves much lower. I would also do fewer shelves rather than more. Try one longer shelf on each side. Too many small shelves has the potential to look visually cluttered.