Hello!
I was recently (1 month) given an out of shape ficus tree. I pruned it of dead branches and got rid of spider mites, and after that it had quite a bit of new growth art the end of the pretty sparse branches but new issues seem to be appearing.

When I got it the pot was full of the tree's fallen leaves, and slugs and woodlice and other critters were living in and on the soil. I got rid of the slugs and the leaves, and pick out the other insects when I see them, I've been told woodlice weren't going to harm the plant so I'm mostly trying to prevent them leaving the pot and entering my flat, and haven't been obsessive about getting rid of them.

The soil is rich forest earth, and is really moist. I stopped watering 2+ weeks ago but it doesn't dry at all.

Some green leaves have started falling and have lots of things wrong with them.
Some are wilted with a black spot extending from the stem into the leaf.
Some are losing colour, as if drained.
Some have dry patches on the side ringed in brown with a yellow 'halo' around.
Some are curly, and a most have irregular shapes.
Most of the leaves are pretty small and thin, and have various shades of green. there are very few 'healthy' deep green regular leaves.

The ones that worry me most turn yellow-brown but don't dry up at first, and when they're fully brown little brown/black dots appear in the leaf. This happens to all the leaves on a branch pretty quickly, starting at the tip and moving inwards towards the tree. The branch itself turns black and wilted and dies.

All the dead and dry branches feel powdery, idk if this is normal.

The tree is sat in front of a window that doesn't get very much sun not far from the heater, but I live in a small flat on the 1st floor and have nowhere else to put it..

I want to change it's soil and start afresh but it's winter and I'm not thrilled at the idea of taking the tree outside into subzero (Celsius) weather to repot it, but I fear waiting until spring would be too long. I don't have a bathtub and my flat is carpeted, so I can't do it inside either.

Will it be ok if I take it outside for an hour? Do you know what it may be suffering from? How do I make up for it's bad location?

I'm sorry about the mega long post and will take any help gratefully, I don't know much about plants but am learning.