I understand this line of argument, I do find it ironic that the left which has always championed making the courts a "free for all" is now arguing against it. Seems to me that there opposition isn't against the court becoming a "free for all", as much as they liked the result of the state court.
I do think the federal interventions so far are potentially problematic, note that I say "potentially", because I think there is an argument to be made on the denial of due process which I do think is reviewable in federal court.
However, the real solution to this is in the Florida executive and legislative branch, they do have the authority to put a stop to this, and they should. I don't believe anyone should be withheld nourishment, absent clear evidence (in writing) of the wish to refuse such, as long as there is someone willing to bear the burden of providing them. I do not believe mere rights of guardianship should encompass the power to deny the ward such, and I do not believe the courts should find evidence of the wish to refuse unless it is in writing and witnessed. All of this can be written into law, and should.
You do know that the Florida Legislature passed the law that allowed Greer to make his decision? They wrote the laws that gave guardianship to the next of kin and allows for the removial of life support (and yes under Florida statue a feeding tube is a form of life support).