Attempting to join 277 and 126 is a bad idea no matter from which angle you look. All you have to do is read up on the Portland, Oregon, experience. The short version is that downtown Portland was dieing even though there was an overhead expressway carrying a multitude of traffic. They tore down the freeway and converted it to a wide landscaped boulevard and, voila, downtown Portland revived and is now quite vibrant.
The HOV lanes are another kettle of fish altogether. They seem to work very well in northern Virginia entering Washington, DC, and a few other spots I have seen, however, I really don't think that Columbia will benefit from them. I have viewed the traffic on the surrounding interstates during rush hour and seem to observe that only .1% of the cars would qualify to use the HOVs. First, they would go empty and all of the traffic would be squeezed into the remaining lanes, then, discouraged by seeing the empty lane, the non-qualifying drivers would just ignore the rules and use them anyway. Thus, IMO HOVs would end up being a grand waste of money here.
IMO, if we are to continue to be automobile intensive and shun high speed rail and other forms of public transport, all of the surrounding interstates for at least 30 miles out from the city should be 5 lanes each way with the fifth lane ALWAYS being exit only. The real moral and long term solution, of course, is public transportation.