20% chance? Seriously, I am skeptical of a neuro that speaks in odds in a situation like this. It doesn't make sense. You simply have not described ALS symptoms. It still sounds like a CYA situation to me. You have been chasing this diagnosis for a while now. You have two choices, let the next three months be 'rough and anxiety filled' or live your life. You don't have any functional difficulties, so I would say spend the next few months enjoying, worrying won't change the outcome regardless. If you're talking odds, an 80% chance that it's NOT ALS is something everyone on here would wish for.
Best wishes,
Tracy