If you foil and the ribs don't have the needed pull back 1/4 " to 1/2" (at the time of foiling) suggesting that the temp was too low or they didn't cook long enough at the temp you were cooking at. Then you will need the moisture to steam the ribs while they are foiled. Not adding any liquid is just going to take longer to cook and you still won't be done using a 2-2-1 method. My smoker 225 for 2-3 hours and I foil, if I had to open the hatch 2 or 3 times then it might be more time. So I'm saying it is a combination of stuff, temp, time, doneness, and the ability to be flexible and adjust when things are not working according to plan.
Regarding foiling and liquid, think braising, when you braise the liquids you add can bring a lot of flavor and the main purpose is to break down the tougher connective tissues on tougher cuts of meat as the meat steams. Braising is for tougher cuts of meat. You don't have to use apple juice, people do it to offset the spicy rubs. I've done beer, the water in my drip pan with some onion & garlic, wine, etc.
Regarding fall off the bone tender, a lot of people think that is over cooked. The desired doneness is tender so the meat isn't falling off the bones until you tear it and then it tears clean leaving no bits on the bone. Its a fine line but you will know the difference once you taste it.This site
might give you the tips you need, its a good resource.