You could let the two players who like empire-building do that between sessions through email, a forum or whatever fits best for you.
Then it doesn't take time from your sessions, and those who doesn't care won't have to bother with it while still getting the benefits.
For the second question I can't really help. Being a relatively new GM I haven't tried higher levels and will have to see how it works out.
The hex-exploring and quests should still be part of what the players do, even if you run the kingdom in the background. Even if they don't claim the hexes for their kingdom, their original charter still charges the PCs with exploring and mapping the Stolen Lands. Also, while the PCs are not themselves running the kingdom, the kingdom would likely still ask them for help when it comes to the kingdom-specific quests, if only because they are experienced adventurers.
Also, +1 on Leonal's post regarding doing the kingdom-building by email for those two players. If they enjoy it, it would not be a good idea to cut that aspect out completely. Since it takes a lot of time out of your sessions, doing it by email would be a good idea even if all of your players loved it :)
As for high levels.. Have you tried running a high-level game before?
I third Leonal's opinion re: taking care of the kingdom building aspect out of game with those interested. There's a goodly number of resources out there that you can now do lots of building and sharing of documents online - forums, blogs, wikis, even email. It's how I'm planning on doing it with my group (I've set up a wiki for the campaign, and will be using it extensively when the kingdom building aspect comes up). Like you, we're very much limited in time - moreso, since we not only do things in a limited time slot (roughly 3 hours for us), but we can also only get together through IRC chat, which takes for *ever* compared to a live session.
As to your whole question of high level... I suppose my first instinct is to ask what you're afraid of in terms of high level? Since what most people (by which I mean DMs) dread about high level is the prep time, the fact that you're running a premade adventure kind of rules that out as a concern... I dunno. Anyway.
I wouldn't want to have my players miss out on War of the River Kings. Just cut out all of the stuff about the First World and you'll be fine. And the war is a suitably epic ending to the campaign, whereas Fort Drelev is kind of meh for a campaign about building and managing a kingdom's needs and desires (though, well, I suppose your players don't much care for that - which begs the question why they agreed to play a campaign *called* Kingmaker to begin with but whatever.)
Spoilers.
My main problem is with Nyrissia and her 51 AC, which is really REALLY hard to hit...obscenely so. I'm looking ahead, and even advancing the stout dwarven fighter with training in great axe and granting favorable combat conditions, I think that he would be hard pressed to land a hit on her.
And then you have encounters with things like the jabberwock, and I just don't understand how characters are going to have a reasonable chance of defeating these opponents.
Now the difficulty doesn't seem to get impossible until Adventure 6. Even in Wrath of the River Kings, the opponents seem to be reasonable challenges - AC's in the 30s or so. The ramping up of difficulty between 5 & 6 just doesn't seem right.
I think you are underestimating the players' ability to get numbers that high. The fighter in my group had +18 TH and 32 AC at lvl 8, though I have allowed limitless crafting (he has good gear, but not insane). You don't have to min-max like a madman, just buy +X weapons and belts of str +Y. But if you have a group that doesn't care for gearing up for combat, you could always reduce the stats of the bosses. As for playing high level: I used to think it was hard, but it's really only spells that get complicated (read up on caster bosses). But if you decide to end at River Kings (which is in itself a good ending), I would recommend pimpin' up the last encounter to end stuff with a real bang.
On the xp-problem, here are some ideas: We solved this by lvling to the required lvl at the start of each adventure if they were lower. If that doesn't work with your group, either throw in much more random monsters and increase quest xp.
Spoilers.
My main problem is with Nyrissia and her 51 AC, which is really REALLY hard to hit...obscenely so. I'm looking ahead, and even advancing the stout dwarven fighter with training in great axe and granting favorable combat conditions, I think that he would be hard pressed to land a hit on her.
And then you have encounters with things like the jabberwock, and I just don't understand how characters are going to have a reasonable chance of defeating these opponents.
Now the difficulty doesn't seem to get impossible until Adventure 6. Even in Wrath of the River Kings, the opponents seem to be reasonable challenges - AC's in the 30s or so. The ramping up of difficulty between 5 & 6 just doesn't seem right.
I think you are underestimating the players' ability to get numbers that high. The fighter in my group had +18 TH and 32 AC at lvl 8, though I have allowed limitless crafting (he has good gear, but not insane). You don't have to min-max like a madman, just buy +X weapons and belts of str +Y. But if you have a group that doesn't care for gearing up for combat, you could always reduce the stats of the bosses. As for playing high level: I used to think it was hard, but it's really only spells that get complicated (read up on caster bosses). But if you decide to end at River Kings (which is in itself a good ending), I would recommend pimpin' up the last encounter to end stuff with a real bang.
On the xp-problem, here are some ideas: We solved this by lvling to the required lvl at the start of each adventure if they were lower. If that doesn't work with your group, either throw in much more random monsters and increase quest xp.
I agree. I think you are reacting to the general powerup Pathfinder characters and encounters have received in comparison to 3.X. AC 51 just ain't what it used to be. My suggestion to you would be to keep playing the AP, and make the decision on continuing as you get closer to the higher levels. I think you'll find, if your players are reasonably proficient at choosing advantageous options for their character build, that their powers will scale up pretty much in tandem with the difficulty of the encounters. My problem, in fact, is going to be more that the PCs are going to become so super-powered by then that it will be hard to challenge them without extensive modifications to the encounters.
All that said, GMing high-level adventures does take much more preparation and is much more difficult, if only because of the sheer number of options each creature or character has available to them. Not everybody likes it. I confess to enjoying the mid-levels more myself. If you and your players don't enjoy high level play or want to continue all the way through, I think it's fine to end the AP after any of the later modules.
A 51 AC at lvl 17 is about equivalent to a 26 at lvl 1. Difficult, but still doable. For instance, your fighter:
17 BAB
4 Magic weapon
8 str (12 is possible)
2 Weapon Focus
4 Fighter Training
=33. Hit on a 18, without any aid or support
Then you can include buffs and allies, which you don't really have at low levels. With 1 round of buffing you can get, with a cleric and wizard:
+2 flanking
+1 haste
+1 Prayer
+4 greater Heroism, 17 min durration, castable ahead of time
Allowing you to hit on a 10. With a bard you could get annother 4 from Inspire courage. And all of this is before any debuffs you can get on her. I don't have her stat block up, so I don't know how resistant she is to things but my group has used many status effects to drop ACs.
Entangle for -2 (new ranger varient can get this if he hits, plenty of spells to do it)
ally grapples for -2 (monk, barbarian, or black tentacles)
Prone for -4 (trip or bull rush)
Using critical feats is also easy to apply some more. Stunned is possible at lvl 17, and you get to apply 2 as a fighter.
High AC is annoying, but stack enough buffs and debuffs on and you're looking at this guy hitting more than half the time.
I think you are underestimating the players' ability to get numbers that high.
This. In my experience, it's I as the DM that has to spend a lot of time and effort to improve the high-level enemies so they won't be complete push-overs. At least, that's how it was when I ran Age of Worms.. The players will always find a way to deal with things :)