So you admit to breaking the rules for a scared and lonely 70+ year old lady as an average member of society.
Yet you think 18/19 year olds aren’t able to do the same.?
Yes, we broke the rules for my MIL. Though they actually did end up changing that rule in the end.
If necessary, and I felt DD was struggling, then yes I would break the rules again for a worried and lonely 18y if she was stuck in one room with no one in a 'household' as company.
If they want to leave, tell them you’re happy to have them home. But how does it help running after them?
Well yes, that's what I would tell her. I wouldn't force her home. But if she wanted to come home I would let her come here. However I would advise her not to use public transport or to start mixing with others in her area. Instead DH would drive and get her if she needed him to (so in car, no public transport, no stops, etc) and she would be able to isolate her - in the same way she would do so if she was 6 months younger and was at school when a 'bubble' burst.
I am clinical vulnerable so I do understand the need to be careful. We would do so - carefully.
Hence DH would collect her if able and that's what she wanted. She would then be able to self isolate in her room, but be able to access her family home with me and dh being more careful around her, but not leaving her totally on her own if she was in a similar situation to the OP's DC.
I am most likely more at risk every single day when I go to work and mix, with no SDing, with almost 300 pupils, than having my own 18y dd in my home.