I can’t demand that my mother does anything differently to what she’s already planned. Any conversation with her about it is likely to elicit a brush off or become about cost.
I'm saying this kindly, @Candlemascandy but maybe think on it?
What comes over is that you're in a stalemate situation. You accuse your mum of not 'bending' (being able to change her mind) but your own behaviour mirrors that too.
You come across as not willing to express your feelings to her (for fear of rejection?)
You use emotive words like 'demand'.
A conversation isn't a 'demand'. It' s an opportunity to express your feelings, without anger. Being assertive is saying how you feel - it isn't the same as being aggressive or 'demanding'.
You're also doing what psychologists call 'catastrophising'- which is jumping to the worst scenario when there is no proof it will happen.
You don't know she will brush you off.
You don't know she won't change her mind.
I suspect you're using those thoughts as a defence mechanism to a) protect yourself in case she won't change her mind and b) as an excuse not to talk to her- which you clearly find hard.
If you won't raise it with her, then you are stuck.
But I'd not want to spend the rest of my life feeling resentment. No matter how hard the conversation might appear, I'd rather do that. Otherwise, it will change your relationship forever, and at 81 she's not got that much time left.
You own behaviour is as stubborn as you 'assume' hers will be.
You've made up your mind that you won't discuss it to give her a chance to see how you feel.
You've also ruled out options like your H looking after your children if you went on your own.
I do understand your feelings but maybe also recognise your own issues that are stopping you talking about this with her?