What are the rules for a flyby grapple?

Question

I'm trying to create an encounter where PCs are climbing a vine or rope down a 100ft shaft. About half way down, a creature flies up and attempts to grab one of the PCs to fly away with her.

The monster I'm using (a sabosan) has a fly speed of 40' (good) and the Flyby Attack feat (if that's relevant). My understanding of the rules is that:

• For round 1: the sabosan just needs to fly to the PC and grapple her.
• For round 2: the monster must make a grapple check to maintain the grapple. If successful, the monster (and grappled PC) can move up to half the monster's speed. Because the PC would be in a hazardous location (50' in the air), the PC receives a +4 bonus on the maintain grapple check. And because the grapple check is a standard action, the monster could only move 20' with the target on each turn (if the grapple succeeds).

Questions:

1. Does the above sound correct/fair for the rules of a flyby grapple, or is there something I'm missing?
2. Also, should/do I need to account for the PC holding onto the rope while climbing down? Perhaps give the PC a +2 to their original grapple check?

Answer 1

1. Yes, but it has nothing to do with flyby attack
2. 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.