It is because this last episode was different than her previous ones that I suggested looking for a different cause. It is common for owners to say the dog stopped breathing during a seizure when they really are still breathing, but it is not common for dogs to go limp. That is why this is different and makes it worth looking for other possible causes, including the heart related ones I mentioned above.
No, an infection of her gums would not likely cause seizures, but that could explain the licking behavior you were seeing. It could also explain the sneezing if the infection is quite severe. Some of the roots of the teeth extend into the nasal cavities so if they are infected you may see some sneezing. Also, more serious but less likely, a tumor within her nasal cavity could cause both seizures and sneezing. You would probably have noticed significant discharge from her nose if this was present though.
The original seizures you describe with the arched back, stiffness, loss of bladder control would be classified as grand mal seizures, so this is not really new. Seizures often become more severe and more frequent as a dog ages - The severity and exactly what a seizure appears as depends on exactly what parts of the brain are being stimulated by the excessive electrical activity in the brain (this is what causes the seizure) - a mini-seizure would be where only part of the body is affected - a facial spasm or twitching of one limb. A seizure in which the whole body is being affected such as the original seizures you describe is a grand mal seizure.
It is good that your vet did blood work on your dog, but that really doesn't evaluate if there could be something going on with the heart. Hopefully he listened carefully to the heart during the physical exam, but even that does not necessarily mean everything is okay. The additional tests I mentioned above to evaluate the heart would be a much better way to more completley evaluate this. If your dog continues to have episodes where she becomes limp as opposed to stiff, I would check into these tests further.
If you are not already doing this, it would be a good idea to keep a written record of any abnormal episodes that you see. Write down exactly what your dog did, how long it lasted, and if there was anything unsual going on at the time in her environment (construction in the neighborhood, particularly active play session, just anything that is out of her normal routine). Sometimes patterns will be seen easier if they are written down. It will also help to evaluate if the episodes are indeed becoming more severe or frequent.
Let me know if you have additional questions.