In order: yes; not very likely; and yes, almost certainly given her age.
So, please: you do very urgently need to see someone as the things you are seeing indicate that if your dog does have SM she is already experiencing pain (and these are very suspicious symptoms and even if this isn;t SM, really MUST be addressed as they do indiocate some serious problems). Limb weakness in particular is considered a sign of rapidly progressing SM and is *a very serious symptom* -- Dr Clare Rusbridge feels that surgery is probably indicated -- or palliative care -- once limb weakness is there because it indicates a more severe form of the disease. At this point it is urgent to take some decision on the dog's future if you care for her wellbeing and do not want her to suffer unnecessarily, which I am sure is the case -- eg whether to consider surgery or to at least relieve her pain, expressed by these symptoms -- because anything not addressed right away will probably not be alleviated by surgery in the future as damage is generally irreversible.
Dogs under three or so that are symptomatic are generally the most severely affected and should at the very least be assessed and put onto adequate pain relief medication (as noted, scratching, yelping and limb weakness are all signs that the dog is suffering and trying to live with ongoing, already-existing pain. This out of kindness should be addressed).
I urge anyone who wonders whether to do anything because their dog doesn't seem to be showing very many symptoms, to please read some of the human SM sites to get a sense of the severe daily pain most symptomatic sufferers must live with (and how vague their expressed symtpoms are -- how does a dog tell you it has a massive headache?). All the researchers working in this area feel that by the time a dog is showing symptoms, even mild symptoms, it is already experiencing compromising pain. At the very least, there are many medications that could bring a better quality of life, even if only in the short term, to any dog with this condition -- a symptomatic dog IS SUFFERING.
The condition is almost always progressive and any dog showing symptoms under age 3 definitely needs to be assessed.