You do it from the little icons, the same as you built the garrison. Once you have the prerequisite levels you get a new icon to put a tower anywhere,even if you don't have a garrison there. Upgrading the building also has a separate interface to the garrison interface, so they're basically separate subsystems, although a garrison benefits from buildings in the same location.
Basically, give all the materials to one of your garrison guys even if it overburdens them, it saves lugging it around with you. When building, it counts materials held by any of your characters in the same tile, and seems to add up everyone's Construction skill bonus too (though there are diminishing returns after your first level 10 construction person).
With the mage, only switch to two shields if they're getting killed a lot, or being a pain in the ass costing a lot with healing. This is generally a good idea near the start of the game for mages and/or priests, or if you bring a new character into your main party who is much lower level than the other guys. Once they're surviving well then you can throw a weapon on them for a few extra hits. But, even at high levels I have a party of ~level 23, and a level 18 mage was along for the trip. I lost count of how often he died in level 4 dungeons. So mages are just death magnets all the time if you're aiming for high level dungeons or city raids. I eventually left him at home and added another priest.