FWIW,
Don't know if you really need a JMT companion. (Having no notion of your trail abilities; sounds like you do know what you are doing though). How is your solo?
Plenty of company at night - hike at your own pace anyway.
The Muir is an adventure, a rolling party, intellectuals and heathens, all playing hopscotch, meeting and passing each other constantly as they all work their way along – the passes. It is not solitude.
You will see/quickly become close to the same 15 – 20 characters for weeks. Believe it or not, can be best if start out alone.
On the trail…”Doin’ the Muir”… an unspoken bond.
- just my 2¢