I'm sorry that your question wasn't answered in a timely manner. There are so many body systems involved, Mimi will need to have a thorough physical exam performed by her vet who then will present you with diagnostic options.
Tail twitching can indicate an unhappy cat. It's usually a sign for us to "back off". If the twitching is uncontrollable, however, it can indicate simple partial seizures or, less commonly, tremors.
Head jerks (tics) are another indication of simple partial seizures (previously called petit mal) in cats.
If the same leg that is lame is the left leg that she's having trouble with when reaching toward her face, that leg may be arthritic at her age and, yes, neoplasia such as osteosarcoma is a consideration. Careful palpation of the leg and spine is indicated and X-rays may be recommended.
The dewclaw growth needs to be directly visualized and needle aspirated by her vet. The aspirate is then microscopically examined by the vet or our pathologist.
I regret that the photo of the facial lesion is out of focus and difficult to see. It's not representative of pemphigus, however, which invariable causes symmetrical skin lesion on the face. It appears to be well-circumscribed and benign.
Please respond with further questions or concerns if you wish.