I am sorry to hear that Princess has had episodes of reverse sneezing despite having soft palate surgery.
Dogs with allergies are generally itchy, they rarely suffer from any sort of nasal congestion or irritation that would cause sneezing/reverse sneezing. They do not have allergies to other pets, although lots of dander from other pets could be a nasal irritant.
Does she have any sort of nasal discharge?
A sneeze or reverse sneezing simply means that something is irritating her nose/pharynx. It can be dust, pollens, an irritating, strong smell, increased tearing draining into her nose, a viral, bacterial or fungal infection or a foreign body, a tooth root infection or nasal mass irritating the nasal mucosa.
Reverse sneezing can be related to gastroesophageal reflux, or it can be caused by excitement or post nasal drip or an overlong soft palate. So palate surgery made sense.
Dogs with dental and gum disease can have lots of inflammation and may be predisposed to post-nasal drip and reverse sneezing.
A reverse sneeze isn't harmful as it's just a spasm of the muscles around the pharynx because of the irritant, but it's no fun to watch. It can be stopped most times by rubbing the throat gently.
If it is because of gastroesophageal reflux then giving her an acid reducing medication may help. You can try either:
1) Pepcid-ac (famotidine) at a dose of one half of 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one quarter of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These will reduce stomach acid and should help decrease symptoms related to reflux. These medications are quite safe and can be used long term if need be.
If this were a one time thing then dust, pollen, a strong smell, or increased tearing would be most likely. Since this seems to be more persistent I would be more inclined to think of an infection (fungal, bacterial or viral), a foreign body (blade of grass, a grass awn), a tooth root infection, gastroesophageal reflux, or mass in her nose.
I recommmend using a humidifier in the room she sleeps in most of the time, and/or taking her into the bathroom with you when you run a hot bath or shower to breathe in steam/humid air can help. That will soothe irritated airways and thin the mucous making it easier for her to breathe.
If she continues to sneeze/reverse sneeze and seem congested the next step would be sedation, dental radiographs to look for bad tooth roots, a nasal scope and skull radiographs to look for a foreign body or mass. It may be helpful to collect samples for bacterial and fungal cultures as well.
There is also a chronic inflammatory process (Lymphocytic-plasmacytic rhinitis) we see in some older pups that responds well to steroid therapy. It is not as common as the other things I listed, and we would need to rule out infectious agents before prescribing high doses of steroids because suppressing the immune system of a dog with an infectious disease could be dangerous.
In short antihistamines are not likely to be helpful, but you can try a few doses of piriteze and see if they help. The proper dose is 4mgs to 8mgs per dog every 12-24 hours. I would start with the lower dose, or 1/2 of the 10mg tablet every 12-24 hours.
Please let me know if you have any further questions.