I absolutely understand your fear. When I was ten, my gran became ill, and eventually housebound, incontinent, and increasingly nasty and uncooperative. She also lived in a 3rd floor flat with no lift, eventually she simply HAD to move as she couldn’t get out of the house to go to hospital or the GP and my mum couldn’t carry her down three flights of concrete steps.
So I’d encourage your mum to stay in her flat rather than move 200 miles away, and simply wait until she is closer to desperate for an easier life in supported accommodation or a ground floor flat. When she realises she needs you, she probably won’t want to move away so far.
My Gran died when I was 32, so my poor mum had 22 years of ungrateful gran expecting my mum to do everything for her - my advice is SET BOUNDARIES. So be very clear to what extent you will expect her to get help if she is struggling with cleaning, shopping, mobility, personal care, keeping her company, taking her to appointments, helping her to socialise, getting her hair cut, or whatever.
I have a brother but he is extremely useless, and lives overseas. So I know it is all on me and I have tried very hard not to take too much responsibility for my mum, but I love her, and it is hard to say no. (My dad died some years ago - a lot of parents don’t live to become a burden for decades, but you’re right to have a plan in case they do.)
My mum learned from her experience with my gran, but it took time for her to decide what steps were realistic. Similar to your DF talking of hanging himself, when my mum was younger she’d assume she wouldn’t be any burden - she talked of me “sticking her in a home and forgetting about her” or eating a large quantity of a food she knows she has an allergy to and “ending it all” when she becomes a burden. Now, however she is in her mid-80s and determined to live forever! She has moved to a small property very close to my house, near a bus route and a supermarket, she has learned to use the internet and is as little trouble as possible, although she does need more and more help now as her mobility is limited. I have researched some lovely little assisted-living flats in case she needs more help in future, that are near my home (and turn over frequently as the occupants don’t tend to live long).
My point is, your parents thinking will probably evolve as they accept the reality of old age. It might be really stupid for your mum to move 200 miles away but she must have a reason -Perhaps it is in her mind to be far enough away that she isn’t ever a burden on you. Are you able to have that conversation really frankly, and ask her, and explain you’d prefer she stayed nearby? It is a heck of a lot easier to visit and help someone who is nearby.
Your dad seems a lot more like my mum in his thinking, that horror of draining someone else’s life away through decades of caring. BUT in my case I do want to help care for my mum, so over the years I’ve made that clear and together we’ve made a plan for how to cope with different eventualities.