66 - she is a mere young thing!
There will be house alterations that can be done to make it less dark.
It sounds as though she thought she would move near you (even though already near her son) and that, as the daughter, you would make her life happen.
SHE needs to make her life happen - it is not down to you. She needs to get out and meet people, join things, volunteer, get involved. She is a grown adult and she must deal with her own life - you cannot take responsibility for her happiness.
I am grandma age and have spent years nursing a sick OH, been widowed and moved house. I do not expect my family to be my whole life and take responsibility for me. I am responsible for myself. They were by my side during the recent bad times, but I now have to get on with life. I am thrilled when they visit or invite me over or out with them, but those are bonuses in my life, not my whole life. And I look for opportunities to help them - e.g. picking up children from school.
I know this feels very hard, but there need to be some ground rules. It is reasonable to nurture her a bit through the first weeks in a new home (unpacking, putting up shelves etc.), but she needs to understand how much you are able to do. And that you have your own lives.
I have good reason to understand how hard it is for someone who has essentially lost their role. I was a student, then a worker, then a mother, then a worker again, then a carer to my OH - and now who am I? I am whoever I choose to be - I have to deal with this transition and make a new life on my own with my DDs behind me but not responsible for me.
I run a choir, sing with another, walk every day, run a book club, am a school governor, join in the pub quiz, am part of a group of women who try and do helpful stuff in the village etc. etc. - and I am older than your Mum.
It is not easy but it has to be done. How you make your Mum take this on board I do not know and I wish you lots of luck. You may have to be gently firm.