I installed my F-A-C G4 Elite on my new truck yesterday (also had it on my 14 but left it on when I traded it in).
You do not have the open the tailgate when you open or close the rear panel, and you do not have to open the rear panel to open/close the tailgate. I think the website info you posted is for the other tonneau cover they make, not the G4 Elite. If I remember right that one has a tab that goes into a bracket on the tailgate to "lock" it. The G4 Elite doesn't have that since the 2014+ have locking tailgates.
Also, pjb0422, do you make your rear seal flip out flat on the top of the tailgate when closed? I always left mine curled under on my 14 and my 15 seems to be the same, I never had any issues with water getting in. On my 14 I could go all the way around with a flashlight from the outside and when looking from the inside you would not see any light coming thru.
Now, for my 15 here is what I have noticed:
I have the factory spray in bedliner, not sure if it makes a difference or not but the front panel seal doesn't seat as "tight" and I can see some light shining thru, not much but I see it. I tried adjusting the panel to lock lower for better seal but at that point I can only get 1/2 on the locking tabs, they don't engage all the way (still locked but only on 1st click instead of both clicks). Also, the center locking tabs are not as tight on my 15 than they were on my 14 so the panel has a little bit of movement if I go inside and push up (have to give it a little bit of force so I don't think it'll be an issue). Those are just some stuff I noticed from my install yesterday.
Oh and if you have a drop in liner you don't have to cut it, you can put it over the bracket, it's pretty low profile, I put my BedRug over it, didn't cut it around the bracket.