I've just finished the game and can answer some of these:
-Replayability is slim to none since choosing different dialogue options doesn't change anything in the story.
-To answer Gavindale's question, sure you could do that, I guess, but all that would change is how you deal damage to enemies. Sword remains your best option but I suppose you could go with another weapon type if you like longer fights.
-Re: lockpicking, you don't get the opportunity to level the skill deliberately, you get given the lockpicks early in the game and there is a limited number of locked chests you can open with them. None contain anything essential, either.
-You definitely can't max all stats: you get a total of 45 points to distribute and that's it, and that's only if you reach level 30 (which will require a bit of grinding). I was level 28 when I finished and I simply followed the story.
-Skills are improved through use, but only if you perform the killing blow with that skill. For example you can wail on an enemy with hand-to-hand skills, but if you switch to a sword and kill it with that at the last second, only your sword skill will increase.
-To check skill progress: Click the quill icon, then select Skills where you see the words "Stats/Skills".
Unfortunately for Revenant, the game's budget was slashed thanks to Dai-ka-fucking-tana screwing the pooch and
Eidos attempting to plug their money leak by reining in the budgets of their other in-development games.
It's really a pity, Revenant had a lot of potential and deserved to at least be completed properly. As it stands, the last quarter of the game is a mess and the ending is... ugh.