Hello and thanks for continuing your research into this very important question!
Some seizure disorders have a cause, such as previous illness (diabetes, kidney or liver failure, hypoglycemia), toxins, tumors or physical injury (like being hit by a car) that has damaged the brain.
Some seizure disorders are what we call "Idiopathic" (sickness without a known cause). Sometimes a dog will have one seizure, and never another. Most dogs with simple epilepsy can be well treated with drugs. Epilepsy seems to be hereditary in many breeds, and usually arises within the first two years of a young dog's life.
Seizures can be brief (sometimes there is not even loss of consciousness), or prolonged. They can be mild in their appearance or physically violent. The more vigorous and the more prolonged a seizure is, the more dangerous it is to the brain.
An older pet experiencing a violent seizure, or repeated seizures, is more likely to be harboring a serious health problem, such as cancer, brain hemorrhage/tumor, or organ failure.
A pet that has seizures that do not affect consciousness often pant quite a lot and appear distressed, but have no willful control over their limbs and posture...they may scrabble the legs against the floor in attempts to stand up or walk.
***Diagnosis can be as simple as running some blood tests, or as complicated as undergoing an MRI to find a tumor or other brain-threatening condition. Deciding what tests to run, or whether to seek the opinion of a Veterinary Neurologist are very individual decisions that depend partly on your pet's ability to undergo the testing and partly on your family's finances.***
Since your regular vet has helped you to rule out major organ failure, you may want to consider that these episodes are some form of hypoglycemia, which is commonly seen in the tinier dog breeds...this would be more likely if she likes to eat a few meals daily, with long breaks in between, or if these episodes can be associated with periods of exercise (say, 1 hour after walks, play, or yard-time). In addition to providing her your presence, you can try to carefully give her some clear Kayro syrup to see if she recovers more quickly. Offering small meals more frequently through the day can help to eliminate hypoglycemic episodes.Here is a link to another article about seizures in pet dogs: http://www.canine-epilepsy-guardian-angels.com/CanineEpil.htm
Severe arthritis of the bones of the neck, or other medical conditions affecting the nerves inside the neck bones can lead to periodic episodes of weakness in the legs...your Next Diagnostic step would include some xrays of her spinal cord to see if there is any pressure on the nerves that should be addressed. Usually, you can see some tremors in the large muscles of the legs as well, and the pet appears "wobbly" in general.