I heard that you can give your henchmen potions if they're dying. I tried clicking and dragging a potion onto my henchmen's portrait but I'm not sure if this worked. If I have a rogue as a henchmen how can I get him/her to disarm traps and stuff...sorry for the nemb questions I just started playing :p

There is an option in the radial menu (at least in HotU) that gives the option of telling your henchmen to do their thieving stuff. Normally, except if you told them otherwise, they will disarm every trap you meet on their own if their skill is high enough. For opening locks, they will go to the locked object and open it if you try to open it and it appears it is locked and their skill is high enough.
Hope this is clear enough, otherwise let me know and I'll try to be more explicit.

Yes you can drag potions on to them if they're wounded, but not during combat. It doesn't have to be a healing potions either, it can be any barkskin, strength, intelligence, whatever.

Unfortunately, sometimes it gets a little buggy and they won't take it, when this happens, I found if you move a long a little bit, and they follow, try again, and they'll usually suck it down.

If its SoU or HotU you can also put potions in their inventory, and they'll drink them on their own in combat.

For the thieving stuff: in the original campaign, start a dialog with your henchmen and talk about their tactics (if I remember correctly), this should also work in SoU and HotU.