Grayson Islands

Do additional skills consume only skill points, or do they also consume abilities?

Only skill points.

Nebos, if I take a Rogue, can I stash one lockpick for myself? Like in the Scrolls.

No. You've been searched everywhere, and they've looked in every nook and cranny.

Can bonuses from a faction or cannibalism raise a stat higher?

Not from a faction, but yes from cannibalism.

Added 2 minutes later
Just keep in mind that it will only increase from the enemies you kill in battle yourself. If you find a corpse killed by someone else and eat it, there will be no effect.