Hello there,
I purchased a Ficus Ginseng in december and everything seemed to be fine until the start of july: I pruned it at the start of spring and it growed well with lots of new leaves then everything started to go bad all of a sudden:
It started to lose almost all of his leaves in a couple of days and I've noticed that he had also a strange white mould on the trunk.

It now only has leaves on one branch and a couple of others leaves in different parts of the three.
I think the problems started because of overwatering and bad soil drainage due to a bad pot and underpot.

I have proceded to prune the worst branches and change the dish pot, now I want to do everything that's possible to avoid my tree from completely dying, so I got some questions for you.

1) Should I repot it even if it's summer?
2) In that case what pot and soil should I chose?
3) What Should I do with the white mould?
4) Do I need to check for root rot and do something about that? How can I?
5) Do I need to prune the tree more severely? If yes which branches should I leave and which ones should I keep?
6) How often do I need to water the three afterwards? Should I water it from the soil or from the pot dish?
7) Should I keep spraying the surviving leaves and branches or not?

Sorry for the large number of questions but I'd try everything to save my tree.
Thanks in advance for your help.
I'll attach pictures of the tree as it is now.

Welcome!
Please take a look:

www.bonsaiempire.com/tree-species/ficus

I've read it all already but it does not have an answer to my problem, unfortunately.
Can you give me more precise answers?

mexiod wrote: 1) Should I repot it even if it's summer?

It's a house plant so yeah, if it is necessary, you can repot it.

2) In that case what pot and soil should I chose?

A pot that fits. You can use potting soil for house plants.

3) What Should I do with the white mould?

Water less, let the soil dry - but don't let it dry out completely (and that is the 4th time this week that I'm writing that on this forum).

4) Do I need to check for root rot and do something about that? How can I?

You can do that. Google 'treat ficus root rot'

5) Do I need to prune the tree more severely? If yes which branches should I leave and which ones should I keep?

Can't advise you on that. This is a forum for bonsai, not for house plants.

6) How often do I need to water the three afterwards? Should I water it from the soil or from the pot dish?

Google 'care guidelines for ficus ginseng'

7) Should I keep spraying the surviving leaves and branches or not?

With water? No, not really.