A- OK, so now you have a FP only line. What about the people who don't show up within their time window? Are they out of luck? Or do Anna and Elsa now have nothing to do for a few minutes whereas they could be spending time meeting another family who weren't lucky enough to get a FP? Does the family now lose out completely or do they get to come back later, and make it a shorter time window for some other poor family? Furthermore, where would the families wait for their return window to come up? Most people loiter around the attraction their FP is supposed to come up for if they show up early, and if they're not in a queue space they will more than likely clog up the immediate area and walkways which could become a fire hazard and all around nuisance.
B- Again, not all families preplan their vacations or do their research before they head to the park. Not the wisest of all decisions, but it happens. So now FP is already given out for the day, and the poor family from Guatemala (who paid the same to come to the park mind you) is just out of luck because they didn't RUN to the location first thing in the morning? Also, wouldn't this once again turn the location (which seems to have calmed down a bit as of late) into the running of the bulls at rope drop? Or what about the folks who arrive after having lunch and decide on a whim that their kids' wish to see the royal duo is worth waiting for? No luck, bub, should have shown up at opening just to come back 9 hours later.
There's a reason there are no FP only attractions, and there's no way that it would be a plausible operation for a character meet and greet either.