M.C.
Yes the spray could be causing her stuffy nose. So could the allergies.
My son is on shots, I was on shots. The need for shots depends on the extremeness of your allergy. There are times when OTC stuff isn't strong enough. My son has had success with the shots and is down to 3 items from 7+ out of 30 that he still has a problem with. He has been on shots for 7 years (I think. I've lost count). He also has asthma and we have him down to one daily medicine for that, plus his rescue inhaler for when he gets a cold. He gets retested once a year.
For the shots schedule, we went 1x a week each weeks for a few months, then every other week. The 2x a week is usually just a suggestion to help you reach the maintenance level faster. Check with your insurance about the cost of allergy shots. We have a copay with ours, so just double check before you sign up. We also had to pay a portion of the serum, which can be $300+. At one point our allergist was charging over $1000 for the serum which he has everyone have 2 bottles of. One for just the pollen and one for anything else.