You can sleep almost anywhere. There are abandoned houses in Aleroth (and Verdistis IIRC) where you can sleep in a bed, or you can use the one in Joram's cellar where you start the game.
You can combine 2 stacks of straw to make a hay bed (if you combine them on the ground you can not move it afterwards, but it can be made inside a container or directly in your inventory). I think you have to drag and drop a bundle of straw onto an unbundled pile of straw, or something like that (click on a bundle of straw to unbundle it, and vise versa).
Once you use the healing gem there is no way to get another. In one of the abandoned houses in northern Aleroth there is a pile of crates where you can find a magic mirror and a book describing how to use it to duplicate the healing gem.
I think I started the catacombs on level 3 or 4.
Actually, even without doing any of the quests in Aleroth or leaving town, it is possible to reach level 4 before you enter the catacombs.
Explore / loot around Aleroth and kill any rats, chickens or cows that you want (50 exp on level 1), but you may want to delay completing any quests. At level 1, if you defeat the zombie Jake (the graveyard would be easiest), you will get enough experience points to bring you up to level 3. This requires a lot of hit and run with a bow, or a moderate amount using Meteorstrike. Defeat him a second time (in his cellar) and you get to level 4.
What 2 locked tombstones? If you mean the two in the centre of level 3 in the catacombs, you need to open the 4 outer tombs before being able to open either of the pair (you can only open one, but they both lead to the same place on the next level).
The passive Stun skill helps with melee attacks; I didn't bother with any of the elemental damage bonuses, but maxed Stun. Poison Weapon can be used with a melee weapon or a bow (archery is quite effective, especially combined with Poison Weapon).
Archery as a warrior - my thoughtsHow have you distributed your stat points?
In your stats window, move your mouse cursor over your offense and defense to check your chance to hit and be hit. If you are missing a lot, or getting hit often, you may want to get you agility up some more.
Speaking of agility, while you were in Verdistis, did you visit the cellar of the Ducal inn? Finnigan there has a quest for you...
The magic lockpicks he asks you to steal are unbreakable, and give you a +20 agility boost while in your inventory. If you started the game in the latest version (ie after installing the patch or in an already updated release), you can return the lockpicks to complete the quest, and then buy them back.
BTW, there are separate forums for Divine Divinity and Beyond Divinity.