No. Two different concepts; plasticity really is more about a kind of ''understudy'' capacity.
Unrelated. We use the concept of brain plasticity to reflect our understanding of observed differences in kids/adults. A 1 year old & a 50 yo can suffer the same insult to the brain and have quite a different outcome. Since the immature brain has not formed all its connections, the 1 yo has a greater "plasticity" to form connections that bypass an injury. The 50 yo does not. It has nothing to do with autism.
No. Both are "variations on a symphonic theme" and are treatable by the use of bionutrients. There is evidence in the medical literature that they are on a spectrum of severity caused by damage to mitochondria, the engines of the cell. This would suggest that there is not enough plasticity rather than too much.