The back row of the whole amphitheater (including the Fantasmic Package area) is reserved for people using wheelchairs and/or
ECVs. The back row is the area you enter on and the amphitheater is sloped down toward the stage. So, if you are in the back rows, no need to climb stairs.
The arrangement in the very back row is a few empty spots for a wheelchair/
ECV to park, then a seat for one person to sit next to the guest in the wheelchair/ECV. If you have a wheelchair or ECV, the rest of your party will be sitting in the 2 rows ahead of the back row - sort of directly in front of the person using the wheelchair/ECV. A usual party is considered to be 6 (5 plus the person with a disability).
The 2-3 rows in front of the back row are for the people traveling with the person in a wheelchair/ECV or for people with special needs (in your case, can't climb stairs).
If you are not using the package, you enter the area with all the other guests and stay in the regular line until about where strollers are pulled off for parking. At that point, a CM will get your party and lead you to seats (since there is only wheelchair/ECV seating at the specific areas, they make sure your party is shown where to sit). If you don't have a wheelchair/ECV/ walker, etc. but have a Guest Assistance Card, you would show it to the CM when you enter and also at this point.
We have been to WDW in some very busy times - including Spring break. We usually get into the line soon after the gates open for the line to start moving into the amphitheater (about 1-1.5 hours before the show starts). We have gotten there as late as 15 minutes before the show and still gotten a place and gotten there as far ahead as 45 minutes and got one of the last places. General busy-ness in the parks doesn't always translate into busy for people using wheelchairs and ECVs.
The Fantasmic Dinner package seating is the far right section, as you face the stage. The show includes some some portions that are movies on screens made of water mist. There are 3 of these, so each section of the amphitheater has one that is directed toward it. The action does take place all over the stage and there are also portions that take place on boats, which start off stage at the right and going around the water to off stage at the left, so they pass by every section of the seating.
The wheelchair accessible seating for the whole amphitheater of Fantasmic is the same - it's the whole back row and is arranged the same as the seating for the rest of the theater, except that there is a ramp on the far right side as you are facing the stage. So, no stairs to climb if you are sitting in that first section.
I don't know if this will work - it's a really good map made by a poster called Robo:
Here also is a picture of mine that shows the back row after the show, looking down toward stage.