Awesome news on the 2nd trip! We might be down then too, but I haven't decided yet.
I totally think that you could take Aria on your own, I'm going to take DS solo in Oct, so if I can do it, you can too!
The one thing you might want to think about is jumping on those point from BLT. The reason why I say this, is because juggling a stroller and a baby on a bus will be very difficult. It will be near impossible to hold her and fold the stroller, and tough to get both of them on the bus solo. I guess you could maybe put her in a back carrier, then you'd be hands free, and it could work, but yikes, that seems like a lot of work! not to mention that you'd have at least one bag with you for diapers and such.
If you stay at BLT you could monorail to both MK and Epcot, and you could get to HS by walking out the international gateway and taking the boat (can you tell I've put some thought into this

).