There is a very good chance that such a rumor is indeed accurate.
New AA should be fully capable of garnering 50% of the DCA market with the 55% of the slots that US has right now but the cost will be to the small cities US serves from DCA and those where US' presence is so small to not have a meaningful difference in the amount of traffic carried. Despite the political influence those small cities might have, there is a limit in the size of AA/US' operation at DCA that DOT/DOJ will enforce. Parker is trying to push the number of slots he can retain but ultimately AA/US will have to divest.
It is also noteworthy that the Brazil route case in which both AA and US are asking for access - along with DL - has also ground to a halt. It would appear that the DOJ and DOT are not going to move forward with the Brazil route case knowing that AA/US will be approved with conditions; giving even half of the new frequencies to AA/US who already combined control more than half of all US carrier slots at GRU is simply not going to happen anymore than AA/US will retain all of its current DCA slots.
It isn't a given that other legacy carriers will be excluded from the process, esp. if they can help connect cities that otherwise would lose service or which low fare carriers wouldn't serve. MIA might be a very good example.
AA/US has the choice to part with a smaller number of slots at DCA in a manner in which they have control and can pick the outcome or they can be forced to divest slots in a manner in which a much lower CASM carrier such as B6 or perhaps WN gains a high percentage of the divested slots and thus have a greater economic impact on new AA's ability to compete at DCA.
The more slots AA/US retains at DCA, the greater the likelihood that the DOT/DOJ will dictate the terms of divestiture and make it a blind transaction such that AA/US have no control over who gains the divested slots.
It is also possible that some of the slots will be placed into a pool that can only be reallocated for small town service, eliminating the argument that low fare carriers will just transfer the slots to serve large markets. And of course, the last thing Parker wants is low fare carriers in AA/US' primary markets at DCA. The DOT/DOJ is just as interested in making sure that big markets have meaningful competition to keep the prices down as they are in making sure that small towns are served. The recent WN route case shows that the size of the awarded market - Houston - was very significant in the route case.
Given that B6 and WN both have enormous potential to build out their route systems at DCA and connect them to other key points on their route systems, the chances are very high that AA/US will both lose some of the small town frequencies they have plus see increased competition on some of their biggest routes.
The DCA and Brazil route cases are probably indeed connected and might also include other merger-related transactions including terminals at US airports and slots at other airports around the world.
It is doubtful that new AA would be forced to reduce its slot holdings down a level that would allow them to operate on one concourse so the connection between the two concourses is still a genuine concern.
The issue with a regional terminal will be the same; unless it is physically connected to the existing concourses behind security, there will still be bussing which involves higher costs and a perception of a less convenient hub.