1) I would imagine that if a grappler got tripped he would instinctively let go to catch himself from smacking his face on the floor.
2) Definitely he would fall to the ground from the ceiling unless he had fly or antigravity or some such effect.
That's a houserule kinevon.
Note: Our ruling was 1) The grapple is broken when the Grappler is knocked prone and 2) The opponent falls from the ceiling.
That's how I would've handled it. Either that, or the victim (choker) can choose one or the other. And if he chose to release the grapple, he'd still be prone on the ceiling and take all typical penalties, have to "get up (down?)" and so forth. And if he chose to fall rather than realease the grapple, he'd land prone if he took lethal damage from the fall, as per the falling rules.
Also, I'm assuming the location of the grappled PC was on the ground, and not like...lifted up into the air being choked out as if danglng from a hang man's noose.
If the PC's weight was being held up by the choker...that would change things.
There is nothing to make us believe tripping ends a grapple. All parts being used in the grapple (usually hands) aren't being used to 'stand' simultaneously.
I wouldn't even rule that they choose to stand or grapple. If a human were tripped, he couldn't say "I let go, so that I can stand on my hands...". Just doesn't make sense. If you trip it, it's tripped, but it is still grappling with all the limbs it was always grappling with. The limbs used to stand, and limbs used to grapple are completely different and interdependent.
With a sufficient fall, you may provide a chance to lose grip, but I would do it in providing a bonus to the opposed grappler on the next 'grapple check'. Both characters would take falling damage if applicable. I would give a +2 bonus for every 10 feet of falling damage taken.
1) What happens to a Grappler should he be tripped,
I would say:
He falls over. Because he is holding on to somebody else, he gets a bonus to his CMD. If he still fails, the grappled person also suffer a follow-on trip attempt as if from the grappler. If they resist, they get a free attempt to break the grapple (he falls, they don't), if this fails the tripped person is not tripped (he stays up by hanging on to the person he is grappling). If they do not resist, they are dragged down as well (he pulls them down with him).Complicated? Yes, but logical.
2) What happens when an opponent is tipped on the ceiling?
Gravity works.
By RAW trip does not end the grapple. I don't think that tripping should end a grapple even in reality: for example as a judoka there is no way that I would let go of a opponent I'm grappling just because I got tripped (by the opponent or by a third party).
Dropping from a ceiling could be trickier: I'd rule that you both fall down and the grappler have to make a check to see if he can maintain the grapple.