Ficus Ginseng losing all leaves and white mould on the trunk

[mexiod]

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.

 

[shinmai]

First, I would prepare myself to accept that the tree is probably already on its way to dead. Many, many ginseng ficus die because they get sold at IKEA, Whole Foods. and the like, with no instructions or advice, especially about the fact that they are usually in gummy, crappy soil that holds water and leads to root rot. Frequently the tree ends up fatally damaged before the symptoms become apparent above the soil.
You don't mention where you are, which for future reference is important context for getting good answers to your bonsai questions. That being said, summer is precisely a good time to repot a tropical plant. The type of pot is unimportant, as long as it has drainage holes. The soil is vastly more important. If it were me, I would go with a straight akadama, or a blend like aoki mix if you can find it. Akadama is readily available via Amazon, and it looks like time is of the essence here. The tree does not need organic material in the substrate, and is arguably better off without it. Using an inorganic material simply means being intentional and conscientious about watering and fertilizing. I would forget about pruning for now; if the tree survives, you should leave it as many branches as possible for if and when it generates some new leaves.
The mold near the base is not encouraging. It may be indicative of dead or dying roots below. When you pull the tree from the pot, if the root mass smells bad, and has slimy or mushy areas, the tree is probably toast. If not, I would suggest using a chopstick to gently remove as much of the old soil as possible. It might also help to use the spray nozzle on your kitchen sink to wash some off [this is one of the very few times when bare-rooting is a good idea]. If the large, tuberous roots that earn the ginseng ficus its name are moldy or soft below the soil line, you may be fighting a losing battle.
If you repot using akadama or something like it, water the pot just long enough for the water out of the drain holes to run clear, meaning you've gotten rid of the dust. The akadama will lighten in color as it dries, giving an indication that the tree needs watering. I would suggest putting an ordinary wooden skewer into the soil, and checking it daily. If it's wet, you don't need to water. If it's damp, water the next day.
I don't mean to sound pessimistic, or to be unduly direct, but I speak from experience on this one. The first ficus I ever had met its demise under very similar circumstances. I wish you good luck, and hope you can pull the tree through.

 

[mexiod]

You don't mention where you are, which for future reference is important context for getting good answers to your bonsai questions

You are right, I completely forgot about that.
I live in southern Italy and at this time of the year the temperature range is 23-35°C (or if you prefer 73-95°F), so it's very hot, moreover I need to keep my tree inside because outside it's too windy and sunny (I live at the 7th floor of a building and I only have a small balcony).

The soil is vastly more important. If it were me, I would go with a straight akadama, or a blend like aoki mix if you can find it.

So, do I need to put pure akadama as soil? Should I buy the small grains to do that?

Thank you for your answers

 

[shinmai]

A medium size would be more ideal, because it would encourage more rapid root growth, but the smaller size would be fine. I have a lot of my tropical trees in 100% small-grain akadama and they do just fine.