It was lovely to read about your plans on this special trip you want to create for Megan - of course she should experience all these great things that I'm humbly reminded many of us just take for granted as being easy to do and sweat over things that really aren't THAT important when it comes to a vacation.
Straight up I'd say Oahu and Maui would likely be the best choices for an island(s) to choose because they are the most developed for visitors for quite a long time now and there is A LOT to do, hence more options and accessibility. Kauai and Hawaii (Big Island) are visited more often now and it's possible to fly directly into those islands when once upon a time, a second flight was required.
Before I began looking up specific attractions I decided to see if there was a guide on Hawaii for special needs travelers and indeed, the Hawaiian government has produced downloadable PDF files on each island with specific information about each island (including Molokai) so I think that reading through the guide might be a great starting point. Here is the link:
So for the dolphin swim, I found Sea Life Park in Oahu that has a "Dolphin Encounters" package which I think would work for Megan (the park is wheel chair friendly and can even arrange for van pickup with lift; make arrangements 2 days in advance) and in this program, there is interaction with the dolphins - they'll give a kiss, play and it says "dance" (??) - while participants are on a platform that is in the water but low depth. Megan would need to wear a life jacket, of course. You'll need to contact them in advance regardless, because special needs cases are handled on a case-by-case basis this is just one place, I found more but you will too:
You also mentioned Pearl Harbor and I found this link....it is wheel chair accessible:
Should you opt to visit Maui, whale watching is available from land or by boat, with a wheel chair.
The more digging I did on special needs, the more reference I found to that PDF guide I've linked in above. I think this will help your family plan the most engaging and stimulating experience for Megan. I hope it will be a very wonderful, memorable experience for Megan and your family.
Let is know if we can be of further assistance. Good Luck