Uh... you might want to do a recount. Doing every written encounter will get you almost exactly to level 4 at the end of Brinewall. Random encounters and relationship XP might push you a little higher, but certainly not multiple levels.
Perhaps you made an addition error somewhere? Also, be aware that XP awards are communal, not individual. That would also skew the available XP considerably.
Yes, it is divided among the members of the party. So a party of four would get around 10k each, three would get 14k, two would get 21k, and if a single PC managed to complete the adventure solo they would receive the entire 42k.
Be aware that this is how it works for all Pathfinder material, as well D&D and most other XP-based RPGs of which I am aware.
Out of curiosity do you think a single player complete the path by themselves.
No. It is conceivable that an appropriate character could make it through an adventure or two, but I just can't see any build, no matter how absurdly powerful, making it through an entire AP alive. One failed save, one unlucky critical hit, one obstacle their skill set doesn't account for, and it is over.
But who knows. I'm seriously tempted to grab someone and see how far a Master Summoner can make it on their own. I'd think at least book 2 or 3, baring bad luck with ambushes.
Normally I'd agree with you but Jade Reagent's got the resurection seal + NPC's who can come in as back up. I'm thinking of trying it myself as a gestalt magi/rogue for teh flavour or wizard/summoner for the power.
I'd still find it unlikely. If by "backup" you mean the NPCs are coming along for everything, that is a bit of a cheat (since you are still a party, not a solo). If you mean having them retrieve and resurrect your body if you fall, I'm not sure I would trust them with that. The NPCs as written are... lets go with "less than optimal." They are also supposed to be several levels lower, meaning anything that took you down would have a good chance of just squishing them.
I'd be interested in hearing how your test goes, though. I'd strongly recommend the Summoner version. A Master Summoner could just summon up a swarm of long lasting creatures and have them take out everything while he sits safe at the back. A Rogue/Magi would have to directly confront enemies, exponentially increasing the chance of that one unlucky roll taking you down.