Nightfly
You have been through a lot. I wish you the best of luck.
Your questions are very general. So I will answer in a general way
1. I would focus on loads that you can complete now. Don't worry about the weight. If your able to train and train safety then you should. Start light, add 5 lb each training session, eventually it will get heavy. In the process you will work on and improve balance.
2. Your second question is describing the term neuroplasticity. Basically describing the ability of the brain to learn, grow and adapt. If you give your body or brain an appropriate dose of a stimulus it will adapt. Reapplying that stimulus, whether its barbell training or leaning to play the guitar is up to you.
Good luck