Say I'm a prowling thief (which I am), and say my party is not particularly good at prowling. If they fail a prowl check, does it expose all of us or just the one who failed? What if I tell them to move away & then prowl ahead so I'm separate from them but they're still in my party?

In essence, how do I prowl successfully with a party?

I played a thief with some followers and I personally used vollinger and tollo underhill and they both did just fine. The problem is that they tend to attack before you are ready. I found to stop this and make prowling much easier in general was to prowl in turn based mode. The reason being when you get master rank you can run in prowl mode while they can't so you can get some distance between you and your party before attacking.

Sebastian and weldo rubin both max out prowl too and I assume they would be just as usefull (your going to have to rush through most of the game to get weldo at an acceptable level though.

Some thoughts from an inexperienced player:
1) To avoid your NPCs attacking all and sundry, perhaps try hitting the Enter to bring up the command line in your message window, then typing #back off
I usually find this works for keeping my allies at bay, important when I want only the undead idiot to smack the treasure chest about.
2) Try learning enough magic to cast Invisibility. This would require quite a lot of Willpower, but I wonder if, in combination with the above, it would provide non-Prowling characters a de facto Prowl ability while it's active.

I've not really tried these, at least not in this circumstance.

It does, if I remember correctly. You still are noisy, but people can't see you. You also automatically become visible again if you do anything (which you don't necessarily when you have higher ranks in prowl (expert?)).

Its kind of pointless to put five points in prowl and nearly max out perception if your just going to go the invisibility route. Prowl is nice because you can stay in prowl even in combat or while picking locks. At master rank enemies will often loose sight of you even if you just attacked them. The spell invisibility needs to be recast every time you do something and eats alot of mana.

Thieves are very skill dependent and don't have enough free points to nearly max willpower so they can get fifth level spells. Max prowl combined with max backstab is more than enough to take out every enemy in the game so theives don't really need to go heavy mage or tech route and are free to use some of the lower level ranks in either for a small boost. Untill prowl and backstab are maxed thieves have it pretty rough though.