Mum thinks he’ll die..

How far away are you going? Can you reassure her that if anything happens you will be straight home? Big difference between a week in Amsterdam and a week at a remote meditation centre in rural Australia in this context.

I know this sounds morbid but do you know what funeral director you would want used if it happens. That is the immediate issue and if you can say to the carehome that that is who you want to use they will arrange for your dad to be collected. Nothing else needs to be done immediately. Does your mum have someone nearby who can be with her immediately?

I agree with talking to the home. They will have dealt with this before.

I think the best you can do is reassure her that if he does she won't have to deal with it on her own. I definitely think you should go.

I don't envy you at all. My DF died a few years ago but my DM gets so anxious if we go away that I've stopped telling her that we're going. So anxious in fact that she ends up in A&E.

You've had some great advice already. Do you have any siblings? I'm guessing that you don't from your post.

If you do it might be worth talking through what would happen if one of your DPs died whilst you're away. Me and my DSis have decided not to tell the one that is away. We've agreed on which undertaker to use and will just ask them to take care of our DM until the other Sister returns.

Have you talked through with your DM what funeral arrangements she would like for your DF?

Don’t doubt yourself, OP.
You need a break to recharge your batteries. Go ahead with all your plans and don’t be guilted by DM.
Good idea above, to ask care team to give DM extra support and reassurance.
I guess DM can see that soon she will be facing up to losing her partner and she’s frightened. It’s not an enviable position.
I wonder if you could pay for a bereavement counsellor to chat with her a couple of times before you go off?

Maybe have a chat with the nurse in charge to get a view of how your dad is - they have training in end of life care. Perhaps if your mum might accept some support for herself? Not sure where u are but Alzheimer’s society have links for support.
perhaps with this information from the nurse will help you talk to your mum and perhaps a named person who knows your dad at the care home could be a support to your mum when she visits while you’re away. Talk through with her the steps she can take if the worst happens - again the care home staff could help wi the this.
take good care - it’s important for you to take a break.