There is a website on YouTube called HT - Physio over 50. There are over 400 videos aimed at over 50s. Some are for very active seniors all aim to improve mobility and strength. He is relentlessly positive but explains exercises very carefully and their benefits. There are chair and bed based exercises included. I think you can click for a transcript which may be useful if you miss something though I know you will want to be selective in how much you read. I saw one about arm strengthening which maybe up your street.
My sister has osteoporosis and a painful back. The latest exercises she has been given are from the osteoporosis society all bed or chair bound. I think they also have a video channel.
Good luck.