[The Fifth Horseman]

- if you put stuff in barrels, does it disappear after a while?

AFAIK not.

- is there a time limit for anything? I mean, can I spend as much time on the game as I want? Rest whenever I need to raise my Fatigue?

Some quests are time limited, but you'll usually know when you get one.

- I read that you either have to pick a mage or go for techno stuff. I picked a mage - does this mean I have to sell all techno stuff or is there a way to use the ingredients to create items anyway?

Being a mage does not prevent you from learning schematics. You might require some books to boost your tech skills in the neccesary areas, though. IIRC they can be bought from the library in Tarant. (note that it doesn't work like in Fallout - books don't give you permament skill increases, but rather give you a boost to the corresponding tech skill for as long as you have them in your inventory)

Are there NPCs that will let you craft those things?

There are NPCs who can craft certain items, IIRC. Don't remember how exactly it is done.

I was personally sad to discover that you couldn't be a mage AND be a herbalist. How is herbalism a tech skill???

Can't? How come? My elf mage had two of the basic herbalism skills. Making healing salves en masse was very useful on the beginning of the game, and as the ingredients are either very cheap or just laying around almost everywhere, selling the surplus also produced a nice income (very useful at the beginning of the game!).