Earlier quoted context omitted.
You are seriously mistaken. At abient temperature, DNA is very stable. Even in an active environment as a human, it keeps quite stable for about 100 years. Isolated for multiple millenia. It is the reason we can sequence neanderthals etc nowadays. It wont be stable for millions of years but with some redundancy you could easily make it to 100.000
I'm not sure this is correct. Maybe if you completely isolate it from radiation. The human body is constantly repairing DNA damage.
DNA in isolate is pretty stable.