Aquatic combat, grapple and waterwalk

[ZatriX]

I'm not sure, maybe this is fairly simple, but I am getting confused. Here's the situation:

A party member is waterwalking across the body of water. He is being attacked from underwater and grappled with intention of dragging him down. The spell has been cast some time ago.

Will this

If the spell is cast underwater (or while the subjects are partially or wholly submerged in whatever liquid they are in), the subjects are borne toward the surface at 60 feet per round until they can stand on it.
still apply to help the person get to the surface or is it only applicable at the moment the spell is being cast?

While on this subject, two more quick ones:

1.What happens if someone tries to bullrush a grapple?
2.How does one pin in the aquatic grapple?

[Surpriser]

You are probably in house rule territory here, as the spell description itself does not give any rules for these "edge cases".

By RAW, the feet of the subject always hover above the surface, but nothing else is affected - so you could drag the rest of the body underwater, with the feet sticking out.

A possible ruling for this situation: It is possible to drag the subject under water with a successful grapple check. However, the grappling creature has to make Strength checks each rounds (assign a reasonable DC, maybe depending on CL of the spell). If these checks fail, both creatures are dragged towards the surface. I would also apply some penalties on the grapple check because it is hard to keep something floating below the surface, even without it fighting back.

For your other questions:
1. You do not provoke an AoO from the grappling creastures for initiating the bull rush. Apart from that, the rules do not really cover this situation. You could have each creature in the grapple declare whether they intend to resist the bull rush and have them make their Strength checks with a penalty to account for the grapple. If more than one creature resists, they might also gain the benefit from being exceptionally stable.
In general, bull-rushing multiple creatures is not covered by the rules, so you will have to define feasible house rules for that.
One other option would be to look into the Domino Rush maneuver of Shock Trooper and apply free trip attacks against bull rushed grapplers (after all, it gets hard to keep your balance if someone simultaneously tries to screw your head off by the ears)

2. Grappling works the same above and below the water (in fact, grappling while flying is virtually the same situation). Pinning someone does not necessarily mean that you push them against the ground or a wall. You can also see it as holding someone at arms length by the neck (the classical monstrous villain move) or restraining their arms and legs with a good grip.