I know this sounds weird but it worked for my Mum and I have seen it work for others in the same situation (I used to work with older people in their own homes) What we did was to leave food around....the kind of thing she could pick up and snack on in passing....grapes, chocolates, savoury biscuits.....anything really that she can snack on in passing and which won't go off over a day or two. I agree its not a healthy diet but anything to get her to eat something while you get sorted....and you will need to get sorted. You say on your way to a diagnosis, can I ask what you mean by that and who is involved? She certainly will need care and support and more than you or any one person, can do even if you lived with her.
I do sympathise, its horrible for children to see their parents deteriorate with this dreadful illness.