Here comes the answer to my trouble: I installed the cane creek spacer and then the stock headset, it looked unstable; then I tried adding on top the stock spacer ring, better but still unstable; so I blamed the stock expander not pulling enough, today I will receive a star nut, but the reality as said above is that the angles don't match. So I will need a cane creek headset or a different bearing that probably isn't easy to find. Again time and money waste  .
This project isn't a breeze, when I built another bike I made everything in one day and half with no issues. Here every day comes a surprise  .

Update: I installed the stock BB with no inner spacer, just to have more chances to remove it later; I plan to order thread in BB, ZTTO or token; if aliexpress is involved is going to take a while.
Chainline: I installed the 52 mm Xt crank, as said before the clearance with the lower frame is minimal, I measured it is 0.8 mm; I think it could stay like this, moving the crank out will harm the chainline: like this is aligned on the 4th gear from the smallest. I think 0.8 mm flex on the chainring will not happen.
Geometry: I put on fork and wheels to work on geometry: my handlebar hasn't much rise, maybe just 20-30 mm, but I feel like adding a lot of spacers under the handlebar, up to 5 cm: is it normal?

The original SC solution doesn't look that efficient either...

I was also thinking about the "problem" of the rear mechanism fender...any ideas are welcome (3d printing?) or maybe just a wrap of rubber or similar.

There was some talk a few pages back regarding what a mud guard for the shock. I had designed and printed myself one a few weeks back that seems to work well. It's held on via a zip and i've attached it for anyone that has a 3d printer.