It's hard to tell (at least for me) what the project is about.
On the one hand the A Gates are a mess and quite possibly the worst RJ experience of any US hub airport at present. Then again, with United Next, the number of 50-seaters that would necessarily need the facility will drop, so it makes sense I guess to get rid of that portion of the concourse. What's interesting is that the new concourse will be designed for wide-body aircraft, rather than just narrowbody/RJs.
The location of the new concourse is indeed where the C/D replacement is/was planned to be. The current C gate Aerotrain stop is designed to have a new concourse constructed directly on top of it, similar to the A/B gates.
Correct, although I would assume that the new concourse will be built with some kind of knock-out panel to allow for an expansion in the future.
Slow clap for United and MWAA. Only took them 40 years, 7 Presidential administrations*, the big CO/UA merger, the unmitigated disaster known as Concourse G, and four different United liveries before they finally got around to replacing the temporary 'Ted shed. It figures that they'll go about doing it in the slowest, most half committed manner possible, but hey: I wouldn't expect otherwise from United, Dulles, and MWAA. It's par for the course.
As for what the worst regional facility in the country is? I think you're right and it is IAD by a hair, but AA's E at Charlotte is quickly becoming loathed-it was great when it was built as a Dash-8 and CR2 facility. It is terrible now that it's mostly CR9s and they have expanded it out to what feels like Concord. EWR A, ORD T2, and Miami's Eagle operations are no great shakes either.
*When the "temporary" C/D was first constructed, Reagan was President, Germany had both a Chancellor and a General Secretary (and two Berlins), the
Soviet army was still in Kabul, Growing Pains was just premiering, Madonna's Material Girl was top of the charts, and new Coke was still a thing.