Its not easy. I think your husband needs to work, and if that means moving, that means moving. He certainly can't not work for a decade or more, because of his MiLs health. Not least he should be building up a pension.
Ways to manage might include:
Look at sheltered options. We live in central London and moving my mother here was not an option. It would have taken her away from networks and London would have been too busy. She is now in "very sheltered" housing which is great. It means she is within a community, and here is someone there 24 hours to deal with an emergency. It also means she should be able to cope for longer, thus keeping her out of a care home. A cooked lunch, weekly cleaning, handyman services, some activities and a helpful reception are there. My mum also has a carer calling in daily, who will do extra hours when needed. For example she is taking my mum fr an emergency GPs appointment this afternoon. Social Services should be able to suggest alternative care. You might ask for an assessment on the basis you might not be around, so she will need to tap into what is available.
Look at contract work, so he is away for short sharp bursts. Not ideal but one way of resolving conflicting needs.
He goes out first and you follow only when he is established. A lower cost country might have good accessible care, so your mum might move with you, at least for the early years.
Find somewhere to stay so you can make extended visits back to your mum. My mum has a two bed flat, and our thinking as the ILs get frailer is that we will rent out of season holiday accommodation if we find ourselves up there for any extended period.