So will any item that I pick up while in CoF will have the “Cannot Be Traded” tag?
I gambled an item at the NPC in the Council Chambers, and it wasn’t tagged as untradeable, so that’s one exception, but otherwise yeah. If you have charcters in MG, they will be able to use any items that aren’t CoF tagged, but they can’t be traded without resonances.
Tagged items just can’t be tradable. So if CoF was responsible for that item existing, it will usually be tagged. Otherwise you’d be farming items in CoF and trading them in MG.
How do tags interact in regards to crafting uniques with legendary potential?
Do they have to match? Can they mismatch? Will a mismatch result in a different tag on the output?
If you make a Legendary with a Unique that doesn’t have the tag and an exalted that does have the tag, the resulting Legendary will have the tag, and vice versa. So no matter which one has the tag, as long as one of them does then the resulting Legendary will.