January is not a great repot time for Schefflera (for you). Summer, when they are active is by far the better repot season for most tropicals though it can be done if conditions indoors are good. Decline may be because it could not recover from the repot through the cooler months.
It appears you were modest with root removal (at least for schefflera) so physical repot should not have caused this. Something post repot is more likely and I'd guess conditions - probably watering. The informal greenhouse may have caused reduced water uptake and consequent wet soil and root rot?
Dehydration is another possibility if you've also erred on the side of caution with watering.
Unfortunately physical symptoms of both conditions are similar so only you will be able to guess which is likely.
The leafless stems appear to be dead. Bark is wrinkled which is usually advanced drying of the wood.
The remaining branch obviously has some life so there is still some hope if you can work out what's causing the problem.
It usually takes a lot to stop Schefflera but you may have managed it in this case. Fingers crossed you can salvage something but please don't hold your breath.
