Yes, to access the run through low level platforms at WAS, people do have to walk through a passageway to access the escalators and elevators that connect to the run through platform tracks. It's not just the fact that passengers may be getting off in DC and then many would be boarding there. Even at NYC Penn Station, Amtrak trains running between DC and Virginia points to Boston have to layover in NYC for at least 10 minutes, despite there being high level platforms and multiple escalators and elevators from the waiting area upstairs to the platforms. I'm sure that there will be time built into the schedule both north and south of DC for these trains so don't be surprised if you see the dwell time at DC a little longer than you might think but not as long as right now.
As for travel from a Odenton to Richmond or any other station south of Alexandria, if the fare isn't more expensive than doing MARC to DC and then picking up your Amtrak train to go points south, I think it's probably worth it to board Amtrak at BWI. You may get more time on your Amtrak train plus if something goes extremely wrong with MARC, you might miss your Amtrak train in DC. It's like a person who lives in Rahway who wants to go to Providence, RI, if you are concerned more on time, then I would say get over to Metropark, rather than take NJT to NWK or NYC.
It doesn't surprise me that MARC is looking at going dual mode. The main advantage of that is just like the Amtrak Siemens Intercity Trainsets, the MARC dual modes would be able to run in diesel territory. MARC should still look into also the Sprinters after more of those Intercity Trainsets enter revenue service.