I'd go about 22 point. Should be OK, but feel free to let Hemlock lend them some goons for the first few sessions. Then see how it goes. It'll depend quite a lot on which classes, how optimised and how experienced the players are.
It depends. One thing you can do is create an NPC who accompanies the PCs and fills a needed gap (for instance, an NPC thief - you can even take the trait to auto-find traps so not to use GM knowledge in trapfinding). This NPC should mostly be a background character who doesn't do anything GREAT, but is adequate instead of a drag. (Mind you, a well-roleplayed NPC can become a vital part of any group, so long as you avoid having the NPC become a Mary Sue.)
Going with a 25-point build is another possibility. I'd suggest however to instead go with a 20-point build... and giving the players double hit points for 1st level (so a Fighter with a 14 Con would have 24 starting hit points). The extra hit points help improve survivability during those first levels when they're most vulnerable.
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If you're into the Mythic rules at all, I'd perhaps suggest allowing the players to become Mythic when they finish off the Catacombs under the town that has the Runewell. One other thing I'd suggest is having an NPC guard helping them out initially - he can die horribly in the Catacombs. ;)
If all the players have is one or two Mythic Tiers, then it'll boost their abilities without being too overpowered. (Do note they have already released a FAQ stating that the Archmage Path abilities should be Standard, not Swift actions for casting additional spells - the same as the Hierophant. This keeps wizardly types from being overpowered.) In the case of a very slow progression, just have the second Trial (and Tier 2) be when they forge a Dominant weapon in the RuneForge most of the way in.)
It's doable, since my group has done a few sessions with only 3 players. The real question is in composition and preparation, if your players play smart they should have no problems with the encounters as they're pretty easy for the most part.
So what classes are the current players playing as?
Don't be afraid to reach out to friends of friends for players as I'm sure there's always someone interested in playing Pathfinder.
I was worried with 3 players, but they rolled for stats. The Ninja ended up with a 34 point buy. Not real worried anymore, they can deal with action economy.
...very, at least in later modules.
That's what the Leadership Feat is for. Or multiclassing. ;)
If I can talk two of my friends into it, I was going to do a two-player Mythic Runelords game, with an NPC middle-aged halfling bard as a background character who helps with some healing and musical buffs (at least until they're powerful enough to not need him anymore).
I have three players in my group right now (see my thread on the progress of the campaign so far for more details). It is reasonably well balanaced (dwarf barbarian, halfling rogue/wizard going into arcane trickster and a human cleric).
I gave them all 25 point buy, max HP every level and I autolevel them at key points so we are not recording XP. This is the first time I've tried anything like this and it works a treat. This may sound excessive but the authors assume a 4 man party. This I've found so far balances it out nicely.
Also the AP tells you precisely what level the PCs should be at each encounter which makes it very easy to close off at a key junction and have them level up together.
This has saved countless hours in accounting and helped progress the game swiftly by focusing on the plot and not numbers.
Hope this helps.