"If you file MFS can you still contribute to a family plan HSA"

It goes by coverage:

https://www.irs.gov/instructions/i8889

"and if over 55 is it $1,000 each meaning you can put in $9,750?"

Into separate HSA accounts. The people are individually over 55, in other words. There is no such thing as a Joint HSA account. For instance, the older person had the HSA all along, and now the younger person also is over 55, so you open an account for them and put $1,000 there.

"Also, can you use Medicare premiums for self-employed health ins and add a supplemental ins as well to box 15 sch C?"

Form 1040.