Oh dear, sorry to hear about all this, it's tough being the sandwich generation.
Lots of people have allergies nowadays, and gluten seems quite a common one. Luckily there are a lot of substitute food that you can buy, and it doesn't always have to be expensive - I recommend dried rice noodles from Asian grocery stores if you haven't them before, very versatile and nice.
As to your father, it's always difficult when you're living far away. You probably can do some things remotely in terms of managing household bills and decisions, but of course not being there physically really sucks. I hope some regular video calls will help so he and your mum knows you are there for them.
Take care
