Keep in mind that there will be lots of traffic directing going on also when you get to DL, so you may not be able to go certain directions.
In May we started out in DCA to see the ELP. we were in Sunshine Plaza. We watched some of the parade, then headed over to DL to get a spot to see the fireworks. We were lucky to get a spot a little ways in front of the entrance to Adventureland. Afterwards, there were several people that were trying to go through Adventureland to get to F! after the fireworks, but it was roped off and the CM's wouldn't let people through. People were being told to go through Frontierland for F!. Several people were very mad about it and got nasty, but the CM's were just trying to do some thing about the crowds. Basically they were trying to get everyone to go in one direction, you could come out of AL, but couldn't go in. Anyway ... we stood there until they opened the entrance to AL and walked from there to see F!. We didn't have reserved seating, but we were standing right behind where the sound boards are, so we had a pretty good view.
We did get to see everything in one night, although we missed some of the ELP, we saw all of the fireworks and F! with pretty good spots for both. However, it is only DH and I, so we have no kids/strollers to deal with.
After this lengthy post
.. you should be able to see all of the ELP and get to F! in plenty of time, just keep in mind the "traffic" directions.
Taking the train is an excellent idea!