Ok I can't help with hotels as I have barely stayed on site (staying at a time share most times I visit). As to the other items:
3. Is it possibly too late too book some of the good park restaurants and E tickets with Fast pass plus considering 4/15 is only 3 weeks out? Would it be worth postponing this trip to early/ mid May so I could be closer to 60 days out and have my pick of the litter?
I would go ahead and book FP for what you can for now, even if it's not your first choice and as others have said, look at it as you get closer and on the morning of to see if you can rebook rides you want more. Flights oP and 7 Dwarves are going to be harder, but you may be able to score something. Just check and keep checking as people cancel or rebook. Others may have suggested splitting the group as you book the FP, i.e., make a FP for 2 of you at 1:30 and 2 of you for 2:00 and you may have better luck. Also, go out and refresh regularly. You get good stuff. Also, April may not be too bad and your wait times may not be awful. Go early, break in the afternoons and come back later. If you don't care for Parades, you may get a good time for 7 Dwarves or FoP, etc.
Restaurants will also have openings if you keep checking. I have found things like Le Cellier 3 days before and the like. Also, people have a 24 hour policy to cancel, so the day before you may get a surprise if someone overbooks or just changes their mind. You can get happily lucky.
4. What restaurants do you recommend to book for my table service meals at the parks? What restaurants do you recommend at Disney Springs or at any of the hotels?
AK: Tiffins is my fav, but Yak and Yeti is decent asian cuisine. I do like the buffet at Tusker house, but it's pricey, even though its all you can eat. The periperi salmon is amazing.
HS, Hollywood derby is my fav. Others have raved about the prime time diner, mostly because of the comedy of the staff. I only at at Mama Melrose once, so it's ok, but not at the top of my list to rush back to.
MK, Skipper canteen is my fav. Good mix of asian food and the staff has the same comedy level as the cruise. They do take advance bookings
Epcot, some of these may be hard to get, but I have had good meals at: Coral Reef, Hacienda St. Angel Inn(the one on the water, not inside the temple), Le Cellier, Monsieur Paul. Ashkerhaus is a good buffet, but you pay a premium to get pics with princesses.
Outside the parks, I have really enjoyed:
Wave and CA Grille at Contemporary, Citricos and the Afternoon Tea at the Grand Floridian, Flying Fish at boardwalk, Frontera Cucina at Springs, Artists Point at Wilderness Lodge. I have a friend who said that Blue Zoo at the Swan/Dolphin is the best seafood she ever ate and others that go to Raglan Road for the irish pub food every chance they can.
I am planning trips to Morimoto and others in the next few trips out there, but don't have much info on those.
I don't do much quick service with my food allergies, but have liked the Kona cafe (sushi bar) at the Polynesian. The beignets at Port Orleans's French quarter quick service are AMAZING and they make seasonal ones. I would only do BoGuest at lunch. I loved it when it opened, but the last few times disappointed me. A quick service lets you see the rooms in the castle and not pay too much for the average food.
5. What are some must see hotels or must see anything outside of the parks?
IF you can do bars, Trader Sam's at the poly is great
Grand Floridian lobby with the piano player is lovely and the Jazz band Outside Mizner's is great in the evenings
Animal Kingdom Lodge does a tour of one of their restaurants that I am dying to try.
Enjoy!