1. On the most part, yes. Sometimes you'll get a weapon or piece of equipment that changes the stats a little (like the Castle Crashers helmets), but they all draw from the base stats of the item type.
2. It seems you can leave the town with 8 items (slots shown next to the store when leaving) and return with up to 10 items (shown at the bottom of the screen in the overworld). It's possible that number changes with 2 players, but that's what I'm seeing with 1-player. I haven't used resurrection or nitro, but I'm assuming resurrection brings a unit back to life and nitro makes your caravan faster for the session or something. I'm guessing all the items have to be used in the overworld, in which case the button reference for each should be somewhere on the screen (I'm playing Xbox version and it tells me on the bottom left that campfire is used with LT).
3. For the caravan, the button controls on XBOX are:
X to mark a point of interest (once for normal point, twice for enemy point, three times to remove), you can place one at a time and when on the overworld the game will point to it.
B to drop a poop, you can use these as breadcrumbs to find your way back to a location later (or just do it for fun).
Y to fire the cannon, though it has a cooldown. If you're playing 2-player, the second player controls the cannon. If you're playing 1-player, it auto-aims to the nearest enemy. The cannon stuns enemies so that you can pass them without going into battle.
4. As far as I can tell, there's no way to look at an enemy unit's stats, but I think on the most part the same class of enemy will usually work the same way. You just have to keep an eye out for different weapon and hat types, but those usually have consistent sizes.