Don't know about the secret menu but yes, F&G will be happening. I'll be there for the week starting April 1 myself. As far as what has changed -
Genie+ is now a thing (paid for FP+) park reservations were a thing but aren't now except for certain ticket holders, park hopping not starting until 2 was a thing but is not any more.
I guess the biggest thing is Genie+. I won't begin to explain it, it's too complicated but there are a thousand and two videos out there on UTube. Frankly, I don't use it but I don't mind standing in line. If you want to get on the new hot rides - Tron in MK - you might want to look into it. Epcot you should be o.k. without it, most people recommend you use it for MK, again, I don't. It depends on if you want to ride as many rides as you can in a day or if you are good with a few and just enjoying the parks.
Some of the Epcot walls are down but not all. I can't remember what was happening 4 years ago (or yesterday for that matter) so it's hard to say. Again, UTube is your friend here.
Last minute ADRs are easy enough to find if you aren't picky about where or what time. I never make them any earlier than afternoon of and have no problems finding somewhere to eat. I'm guessing on your Epcot day you are going to be grazing at the booths most of the day. France, Mexico and Spice Road Tables in Morocco are usually easy to get ADRs. MK, not much to eat there unless you want BOG (not usually recommended any more) or Skipper's Canteen. It's an easy get day of. There is always the option to leave MK and go to the resorts around it, you could always eat at the bar in the CR, it's pretty good. I would suggest looking at
MDE now and seeing what's available. Again, UTube is your friend here as well.
