I think the probability of her eggs being fertile is slim from what I understand of your setup.
If she is still laying eggs she is not properly broody. A hens egg laying cycle switches off when they are properly broody.
You can take a chance and let her sit. You will need to make sure she gets off the eggs once a day to poop, eat, drink and dust bath, so you will have to either leave the coop door open so she has free access, or get used to gently lifting her off the nest once a day and carrying her to somewhere in your garden/yard to feed her.
Scratch isn't really a good enough feed for chickens. You should buy a feed like All Flock.
Ideally her feed should contain 18% protein and given she a laying hen, up to 3% calcium.
You also need to be strict about the length of sit you allow her to sit. Incubation takes 21 days so if after say 23 days no eggs have hatched, you must take her off the nest and destroy the eggs. You can't eat the eggs.
My view is it is very important not to feed a sitting hen at her nest. They need to have some exercise. Sitting hens are relatively easy to handle. They go into a type of trance when they sit and their legs lock. If you gently slide your hand under her body and covering her wings with your other hand, you should be able to take her off the nest fairly easily. She will probably try to peck you.
When you place her on the ground she will probably stay in the squatting position. Put the food in front of her and encourage her to stand. You are also likely to get the vilest smelling pile of poop you can imagine. This is normal. Hens try hard not to poop in their nests when they sit so the poop has stored in the hen since the last time she pooped.
The more often you handle her the more confident you'll get.
One important point is when lifting her off her eggs try not to disturb the eggs.