1. He should change direction. Anything else would seem wrong to me. But, I'm not completely sure.
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With Lemmings 2 changing balloon direction does NOT change lemming direction
so far, if we keep thinking of the Ballooner as an upward Glider, then just like the Glider changes the direction it faces when changing its drifting direction, so should the Ballooner
The Ballooner doesn't
have to be an upwards Glider, it can be whatever we want it to be. Turning around does seem the best way to go, but let's try it both ways and see which we prefer.
2. It should pop. Couldn't it bob forever otherwise?
It bobs around until cancelled by Walker, Shimmier or Jumper. So, effectively, the lemming becomes unavailable and it becomes a matter of strategy whether a cancelling skill is used to "rescue" a Ballooner from the top of the level. Speaking of which...
2) Isn't the ceiling still deadly? What happens in Lemmings 2: The Tribes, where the ceiling is deadly, too?[/b] 
I would expect the balloon to carry the lemming beyond the upper level border, and would therefore expect the lemming to die. 
This would be an interesting restriction, to prevent the player from just assigning the Ballooner anywhere they want.
...the SLX level top isn't deadly, it's modelled on the Amiga L1 level top and its various interactions (doesn't turn Builders, but does prevent lemmings accessing the top of the level). This was done mainly so that Jumpers and Projectiles can continue their arc back into the level, but also because I don't want the top and sides to be deadly in general.
Just to confirm - yes, Ballooners disappear out of the top of the level in L2. And yes, this means that preventing Ballooners from accessing the top of the level becomes part of the challenge of using them. This is worthwhile.
Hence, the decision to have them bob around at the top - they become
unavailable until the Ballooner skill is cancelled, which I think is better than having them disappear altogether; the top of the level is still a threat, but the lemmings can still be rescued - best of both worlds. This means that they can be used in more interesting ways, by both designers and players.
Having said that...
If the level designer makes the ceiling solid by placing terrain all across it, then of course the balloon would pop, just like it does when it hits overhead terrain anywhere else in the level.
...whilst it is important to take potential level design into account, I generally prefer to be a voice saying "let's not worry too much about level design specifics." The reason being that what I've observed tends to end up happening is the conversation gets bogged down in hypothetical scenarios, the overall concept gets lost, and decisions get made which unnecessarily limit the skill's versatility and range for the sake of only one of many possible uses.
we can talk about the "Backwards Walkers" gimmick again
SLX most likely won't be getting any of the skill/movement-affecting gimmicks - for the most part, I think it's a good thing these were removed.
The only gimmicks I think were worthwhile are:
- Superlemming (which has be re-inroduced in SLX as of 2.4!)
- Frenzy (which is available as a "Play Without Pressing Pause" talisman rather than forced on the player)
- Karoshi (which I'm thinking might also be good as a talisman, but since talismans require the level to have been completed, I'm not sure how this would work)
- Wrap (which will be implemented as soon as I can figure out how to move lemmings across large distances without crashing the game!)
- Rising Water (alongside Wrap, we can potentially re-make some of the
Revolutions levels in SLX! It also has the potential to make the Swimmer a more interesting skill. If re-implemented, it will probably be its own object type, and, like in
Revolutions, will likely be triggered by a switch)
Beyond that, I've looked over the list of old gimmicks many times and the others are all mostly very tiresome once the novelty has worn off. So, just making this clear before there are any
actual suggestions to bring these back (I realise your comment may have just been in passing).
