No, it is not circular, it is radiometric and can be measured using different isotopes that all give the same answer, but on different scales of predictable radioatomic decay.
They did not find muscle tissue, they found structures; and for that they are very famous... among paleontologists and also creationists, Dr Schweitzer would be a ROCK STAR to creationists, but she keeps saying that the fossils were just amazingly preserved fossils from fifty million years ago instead of making incorrect grandiose claims.
They've found a variety of elephant species, not just mammoths. As to whether they look exactly like today's elephants is beside the point. There could have been species that went extinct 6000 years ago in the flood, or even after the flood for that matter. They wouldn't have to look exactly like today's species. Creationist believe a certain amount of evolution occurs, just not the massive transformations that Darwinian dogma alleges. Plus God probably created more than one variety of elephants from the start.
she keeps saying that the fossils were just amazingly preserved fossils from fifty million years ago
Of course she keeps saying that. What else is she going to say? She's an evolutionist.
According to this,
http://www.answersingenesis.org/creation/v20/i4/dogma.asp
they don't directly date fossils, but the rocks around them.
This gives an example of how games are played with radioactive dating. It is circular.
http://www.answersingenesis.org/docs2006/0816dating-game.asp
As I said before, mineralization, your first argument, doesn't seem to be a dating method at all.
This discusses the rate at which fossils form under flood conditions:
http://www.answersingenesis.org/home/area/cfol/ch3-how-fast.asp