You won't need a FP+ to enter these queues. For Big Thunder, you will get a return time to come back if you don't have a FP+ time. From what we were told, Spaceship Earth never really hands out return times, so I wouldn't bother with a FP+ there.
For Big Thunder, it should also be noted that if you do not transfer to a Disney Wheelchair, you will have to park the
ECV at the exit and walk in and out, but you will also get to ride twice in a row if they are running an odd number of trains, so that you will come back to the same side as your ECV is on. If you transfer to a Disney (or any standard wheelchair), you can take it down with you, but they will move it to the other side and you do not get to ride twice. At least this was our experience the beginning of this month.