I understand that you are concerned about Molly having possible food allergies and are considering a diet change.
Food allergy is very possible with her as well if she seems to be itchy all year round. Dogs can develop allergies to any protein or carbohydrate so a grain free diet may not be restrictive enough for her.
Even if she is currently only fed one thing there can be something in that food which can be what she is allergic to. Dogs with food allergies tend to lick and scratch their paws, face and ears the most, but any of the "allergy reactive areas" can be affected.
If you are going to make the effort to switch foods make sure that the food that you put her on is a true hypoallergenic diet. The trouble with "grain free", "limited ingredient" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your dog isn't allergic but a waste of money thinking that the food was hypoallergenic and not good for your dog if those happen to be allergens for your dog.
The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food.
Generally what I recommend is trying to clear the skin with a true hypoallergenic prescription diet and then adding one food item (chicken, beef, corn wheat etc) every month to see what they react to. Then we can find a regular food that doesn't have what she reacts to in it to try. As far as permanent diets I do tend to stick with Purina Pro Plan brands or Nature's Recipe as I find those rarely if ever have cross contamination. Purina Pro Plan Turkey and Barley or Nature's Recipe Vegetarian or Venison are pretty good products. I know that this isn't easy from personal experience (my dog is allergic to wheat) and it is time consuming, but worth it.
If you choose to try testing/treating her for a food allergy I recommend that you try a true hypoallergenic diet like Hills z/d or Purina Veterinary Diets HA. No treats, flavored medication or bones while on the diet and it must be used for a least 12 to 16 weeks to see the full effects. You should see some improvement in 6 to 8 weeks.
So in short I find grain free foods are not restrictive enough to be worth it, but a true hypoallergenic food may be. If she is still itchy on Apoquel she may have a food allergy, as those affect a different pathway and so dogs that are food allergic may nit be helped or respond completely to Apoquel. Or she may have a skin parasite or a bacterial or fungal infection that needs to be addressed too.
Please let me know if you have any further questions.