So, how many people can go with the
ECV? There are 4 of us but only one "driving". Is there a bench for all 3 of us?
A 'party' at WDW is usually considered to be 5 plus the person using the ECV or wheelchair.
The wheelchair seating is the entire back row of the amphitheater. There are spots for wheelchair and
ECVs users to park intersperced with a few seats for people accompanying them. The rest of the people from the wheelchair parties sit in the 2 rows directly in front of the wheelchair/ECVs. Whether or not all the wheelchair spots fill up before the other seats is really variable. We have been at Fantasmic when all the spots were filled a while before the show started and other times when they were not all filled when it began.
The dinner package seats are the far right section of the amphitheater and they have the same arrangement for wheelchair/ECV users; it's the back row of the dinner package area, with the 2 rows ahead of that for the rest of your party.
There are a few wheelchair spots down at the very front of the theater. I don't know if the dinner package area has any. I know for the non dinner package, you need to get there early for those spots because there are only a few. Also, be prepared that you may get wet from the water spray during the show.
And, when leaving, it's best to take your time getting out. Just sit until the crowd thins, then leave. The total amount of time from when it ends until you get out will be similar, but most of your time will be spent sitting waiting to leave instead of (very slowly) moving out.