"The question is, are they also ready to to accept that if this is a matter of Federal rather that state jurisdiction the logic of the argument suggests that so are a lot of other issues (as various "medical marijuana" and "gay marriage") many here would prefer to see decided at the state level."
First of all, the issue of “gay marriage” could be decided by a state legislature or by the Congress.
The issue of whether a person can be deprived of life without due process of law has ALREADY been decided by the Fifth and Fourteenth Amendments to the Constitution. The answer is “NO!”
The Congress can act to investigate any circumstances where a person’s fundamental rights might be threatened.
They can issue subpoenas and require parties to appear before them as part of their investigation.
This is a limited power, and in my opinion, it is entirely appropriate in this case.
There is grave doubt that Terri has been given due process here by the state trial judge.
Her federal rights to life, due process, and equal protection under the 5th and 14th Amendments are paramount, and the Congress has a perfect right to intervene to protect them.
Don't forget her right to freedom of religion ( last rights) which her husband has time and again refused to allow.