Attaching offset box?

What brand of insert and which brand of offset box? Without seeing what you have I would guess that to go from the liner to the offset box you would need the same adapter that would be used if you had been able to come straight to the stove.

As far as attaching the offset box to the stove I would suggest getting some hex-head self tapping screws, pre-drilling the holes where you need them for attaching the offset box with the stove pulled forward, then run the screws in to cut the threads, take them back out so you can do your final install and then during the actual install since the holes And threads are now there you can reach in with a small socket wrench and put the hex head self-tappers back in. I would use cement and screws. Just furnace cement and no screws will not be good if you ever have a chimney fire or even a bad back puff.