Mammoths have been unearthed and were found to have died from freezing
while their last meal was warm weather food (namely flowers); does this
support the Hydroplate theory

This is apparently a fact. I am not sure I can answer how it happened.
Perhaps some catastrophic event caused this. Perhaps even a catastrophic
event of miraculous and biblical proportions caused it. However, just
because we have an unexplained mystery, does not mean that this is
evidence for some half-baked creationist theory such as the supposed
Hydroplate theory. Trust me in this, and I have read the writings of
“Hydroplate” theorists–their supposed scientific evidence for the
hydroplate theory does not pass muster as a scientific theory. So yes, it
is true that the thing with the Mastadons is very interesting and as far
as I know it is not explained as yet. It is a far leap from there to
saying that the Mastadons serve as legitimate evidence in support of the
Hydroplate theory. It does not. (For a book which supports the hydroplate
theory, see “In The Beginning,” by Walt Brown. Although I disagree with
his conclusions, it is fairly well written).

John Oakes, Ph.D.