I would just use one of your Fastpass Plus times for those if you want to try it.
As was mentioned, the Fastpass Plus area for Wishes is small. If you don't care whether you see Wishes from the front, the Fantasyland area behind the castle lets you see the fireworks part (but not the castle projections). It is usually uncrowded there and you can walk around before and even during the fireworks.
You do need to be aware that dome of the fireworks are shot off from above the Beast's castle, so they will be traveling across Fantasyland and some areas will be blocked for safety until after the fireworks end.
We used the dinner package for Fantasmic in October 2013 and used Fastpass Plus in April 2014, but saw people entering at the Fastpass Plus entrance who were using
DAS both times. I don't know where they would have gotten their Return Times though (or if they even had a return time), so Fastpass Plus or dinner package are 'surer' options.
For parade viewing areas, you should be able to use/show the DAS to use those. But, they tend to get very crowded and can fill up quickly. And, once you are in there, you have to stay put. Some people gin fit works better for them to just find a spot along the parade route and use other members of their party to 'block/buffer' the person with a disability. Main Street and the castle hub are more busy; if you go into Frontierland, it is usually less busy.