I wouldn't want a warrior to auto-attack (they can rush off towards tough opponents, so there could be a delay before your lead character can catch up), and I don't think mages will auto-cast spells. Most of the time I just had both characters selected, and directed them to move and attack as one (only split them up for tough fights).
If you give the DK a strength based weapon, he will need strength for that. That is why I specified a bow archer as a good fit, since they are agility based. Crossbows tend to do more damage than bows, but are a little slower and have strength and agility requirements.
I almost never play mages in real time games, and have not done so in Beyond Divinity.
You will at least want the summoning doll to use as a mule (summon it, drop all your excess loot in its inventory, unsummon it. when you get to a merchant, summon it again and sell your junk). Summoning dolls can also be used to set off proximity based traps (they can not open containers, so you can not use them for trapped chests, etc).
There is some starting advice in the Killing Fergus topic (even though you are past that point), and more in the strategy guide.
If your starting characters really suck, you'll find out pretty fast. I did my first game (playing on tactical difficulty); it was easy up until Fergus, about 15 minutes into the game, and then not so much. With a restart and some tweaks to my stats and strategy, I did much better.
Melee works fine; some people even played with 2 melee characters. There is no dual wielding, so your choices are one handed, sword and board or two handed. Try whatever you wish. You'll probably have to start off with a one handed weapon, so use that until you find a shield or two handed weapon, and try them out.
When I switched my characters to two handed weapons I left the one handed expertise skills (only put a couple points into them, anyway), in case I wanted to switch back. I had lots of skill points.
I think having a ranged character is helpful, but if you wanted you could keep the DK as melee. The way the game starts you characters are both going to be melee at first, anyway. There is a bow available relatively early, but limited arrows for at least the first hour. You'll be able to get a feel for how it works (and against skeletons it works pretty good). Later, against mages and archers, it helps to have a ranged character yourself (though not essential, and even a warrior can learn a couple spells, even if they don't have the mana to use them frequently).
Yes when you unlearn skills (at a cost related to your character level) you get the skill points back.
There are ways to duplicate gold (hex editing and file swapping), though I have not tried in the latest version, so the procedures may not work, or may be slightly different.
I bought any skills that sounded like they might come in handy, and didn't have any problem with gold. I'm not sure you could train in everything, though.
I had both my character learn the healing spell; it comes in handy. I put 2 points in for each, though probably would have been fine with one.
I didn't level up my summoning doll until almost the very end of the game. Since the purpose of the main characters is to undo the soul forge, it isn't spoiling anything when I say that near the end you can have your DK unlearn all his skills, and have him learn the summoning doll level and skill point upgrades. You may need to scrounge a bit for the gold, depending on how many skills he has then. If you don't want to play in the after game battlefields, though, where it is just your hero and summoning dolls, it doesn't make much difference if get a use from the all the DKs skill points, or not.
There are two ways to exit the first act. Making explosives will give you experience points, so that is the better option. Keep exploring, and you will eventually get to the exit, as well (which will not give experience). There is a hint for the items you need in a book somewhere about gunpowder.
