Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about Misty's increased nasal congestion.
Clindamycin is an antibiotic used for bone and tooth/respiratory infections. It sounds like it hasn't helped much for her.
Gabapentin is a pain medication, and we often add it to other medications such as nonsteroidals for pain or arthritis discomfort.
I would recommend that you continue the humidifier, especially where she sleeps, as that will help thin the mucous and hydrate her airways which should help.
Another option is an antihistamine. Most antihistamines do make dogs sleepy at least initially, but that side effect often wears off with repeated use.
You can try:
1) Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at 1mg to 2mg per pound or one 25mg capsule per 25 pounds of dog orally every 8 hours. Of all the antihistamines Benadryl appears to be one of the worst for sedation.
OR 2)Claritin (loratidine) at 5mg per 25 pound dog once or twice daily. This one is less likely to sedate her.
OR 3)Hydroxyzine at 1mg per pound orally every 8 hours. Moderate sedative effects and it is a prescription drug.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily. Moderate sedative effects.
OR 5) Zyrtec (cetirizine) at a dose of 5mg to 10mg orally every 12 to 24 hours. This one is less likely to be sedating as well.
Some dogs do better on one antihistamine rather than another. You'll have to see which one works. Give each a week trial and if it isn't working try another. There's no wash out period between antihistamines so you can give a new one the next day with out worries of interactions.
You can use a combination of antihistamines and high doses of omega-3 fatty acids to help with allergies. In combination fatty acids and antihistamines work synergistically, much better than either one alone. If her symptoms worsen seasonally I would think that inhaled allergens or irritants are a part of her problem. Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight.
Other reasons for sneezing and nasal congestion are an infected tooth root, a foreign body inhaled into the nose or a nasal polyp or an inhaled fungal infection.
If she isn't much better in a few weeks then she needs another veterinary visit. At that point I recommend sedating her and taking dental radiographs to look for problem teeth, scoping her nose to look for signs of polyps or a mass or fungal infection. They can take samples for culture and biopsy of any abnormal areas.
Please let me know if you have any further questions.