It's hard to tell exact details from just looking at what they've got available online, but I'm thinking that from BWI you'll be able to take the train to last station that is in service, then take the free bus past the closed portion, then back on the train to conference center.
Not going to say specific station names since it's possible that area affected may change day-to-day as they work on it. I'm sure that there will be info available at the airport day-of as well.
I don't know if it will be necessary to by a second pass due to the "two train rides" each way... I doubt it, but even if that is the case, it's only an extra $1.40.
Biggest thing is just to be ready to be patient: Wait for train, go to stop, wait for bus, go to station, wait for train...