Hey Matteo, you have a couple options here...
For those stripped backplate screwholes, you could upgrade to the threaded inserts for all five of them. The screws are available in the Alembic store for a minimal cost,
https://store.turbify.net/alembic/platescrews.html and I bet they'll send you some inserts too if you ask nicely. I don't believe those are listed for sale.
For a quick fix; carefully remove the backplate and screws. Lay it aside out of the way. Use some masking tape to provide backing for each of the five holes, then clip the ends from some wood toothpicks to the appropriate length. Put a small amount of CA glue on each piece and slip them into the screw holes. Once the glue sets, pull the tape out. That should tighten up the holes enough to secure the backplate.
Regarding the nut- replacing it with an adjustable one is tricky. They do this in the shop, and it requires some machine work. One Club Member posted a link one time to a commercially available adjustable nut, but I've long since forgotten where that is... (*if you give me a few minutes I might find it, simply because I remember whose bass it was...) a quicker fix might be to simply place a thin brass shim under your existing fixed nut. If a few thousandths will do what you need, then that's easily executed. I did this after a refret on my '77 Series bass rendered the nut
just a bit too low. I placed a 0.010 shim under it. Problem solved.
*found it... hat-tip KR.
https://www.thomannmusic.com/warwick_just_a_nut_iii_brass_4_str_44.htm