I do agree one food at a time.
Maybe try foods with less acidicy too, sometimes foods that are high in acidic value (like a lot of fruits) can leave a rash because of the acid, not the allergy.
In fact it sounds more like a sensitivity then an allergy, since there are no other simptoms. With an allergy you would also have tummy problems, diareah or vomiting at times as well.
Better to stick with cereals first, then veggies, then fruits.
If the have a reaction from the jarred they should have the same reaction to the homemade, the only difference with stage one foods and homemade, is the amount of water.
Keep the face clean and dry and don't use whipes, as they can also cause rash, mild soap and water, or even plain water are best.