Hi FedUp, I have thought about you a lot this last week, having read 3 separate threads by posters having difficulty with older teens and young adults because they stayed in abusive relationships, unable to protect their Dc while staying, and unable to leave. It was such a huge thing you did, in escaping. The fear that he would see you on the cameras... and reading those threads I was thinking how very much courage you had and how much it will have been worth it, a lifelong thing for your Ds.
Not surprised about the nightmares. The whole chemistry of your brain must be changing. Living under stress produces hormones (cortisol)that affect your brain and thinking, and now you are no longer under constant acute fight or flight.
Very exciting about the guitar! Has he ever played guitar? Have you got a little amp with it? Haha be careful what you wish for, my Ds who started when he was about 8 now has 6 guitars at 18 
. I remember the Christmas we gave him his first second hand electric guitar, he was so delighted. It is such a great thing for them to be able to do, if they take to it.
Are you musical? I am not and while it makes me feel sad that I haven’t been able to be of practical help to my Ds, it makes me even more in aware of how he has learned to play, and develop his own independent interests.
Really sad about your Dad, you will never regret having this time with him. Lovely for him to have time with your Ds too.
Are you all mucking in to cook at Christmas or is there a head chef?