Or would you have to wait until dormancy?

I see no reason to do that job only in the dormant period. When in leaf you just have to be extra carefull.

of course you can, you may need to cut it off not unwind.

I remove any time of the year just before the wire starts to bite int the wood. That is quite often is summer as the tree grows the most during this period.

For most species you will have to.

For many conifers it is common to let the wire sit long, as the wood is very resistant to repositioning. You can then decide to remove when you are ready. However, for deciduous and especially thin barked species you have to remove the wire very soon after wiring. I wire my maples when branches are young, in spring. I unwire 6-12 weeks later. In summer. If they stay on longer the branch has grow around it so much that the scars will remain visible for decades.