- Yes, but it has nothing to do with flyby attack
- That seems reasonable.
When you have the grappled condition, you can't use your normal movement:
Grappled creatures cannot move
AFAICT the only exception is to move as the result of a successful grapple check. In that case the creature can make the check, and move 20 feet, and it can do this whether it has flyby attack or not.
Further, it doesn't actually use its move action to do this; it's part of the grapple action. You're moving instead of dealing damage, pinning, or any of the other possible grapple actions.
There are some corner cases where you could use grapple in conjunction with fly-by attack, but you'd have to move, grapple, and then drop the grapple as a free action to continue moving.
A creature with grab could use it to avoid the grappled condition, and thus be able to move freely. But I don't think they could take the creature with them, so you'd have to stay within reach.
The spell does what it says it does, and the condition (Grappled) does what it says it does. Victor is flung 10'.
The spell moves Victor 10'. Clear enough. Unless something would stop that movement.
The condition doesn't cause Victor to be immobile, it reduces his speed to 0'. He's not nailed to the floor or encased in carbonite, it's just hard for him to move of his own volition. Grab someone by the wrist with both hands: you've got them Grappled.
So, since nothing in Grappled says Victor can't be moved by an effect (and, in fact, as you mention it specifically mentions this effect as one that can move the victim out of the grappler's grasp) and the spell says it moves Victor, that's what it does.
As long as the effect of Thunderwave ejects Victor from Gary's reach, Victor is no longer grappled.
(As @MartinCarney rightly points out, if Gary's an Otyugh, even 10' may not escape Gary's reach!)
Best Answer
Almost-certainly not a comprehensive list, but some issues that come to mind for me immediately:
They are rarely used. Grappling is a fairly ineffective tactic, requiring specialized feats and at best only causing problems for a single target—and there are a lot of ways to get out of it.
The process is very particular. There is a very specific sequence of steps, each with their own ramifications and limitations, which means it’s a very dense “two to three pages.”
“Two-to-three pages” is a lot to look up in the middle of combat when it suddenly comes up. You might not memorize all of the details of a given spell, either, but spell descriptions are usually a paragraph or two.
Grappling is very unlike all other forms of interaction in 3.5e. That makes it very difficult to keep track of, because human brains remember things by association and pattern. All we have for grappling is that it’s different.
Being different also robs us of any confidence or sense that we “get the gist.” It doesn’t match up with our intuition of how things proceed in 3.5e. It may not be counter-intuitive, per se, but it isn’t intuitive, either.
But if we’re being honest, there are things in there that are counter-intuitive. The interactions and conditions applied to each side at different steps in the process have a lot of implications and some of them are not at all what you would expect.
For what it’s worth, I’m still apologizing to the last DM I made look up the grapple rules. We laugh about it, but it prompted some pretty absurd situations, and also took a ton of time to figure out in the middle of a session. It was not good for the game, no matter how much it was the “right choice” for my character at the time.