"Evolution, being a scientific theory, predicts there will be at least 20M years of difference."
Not really. If the design was good enough for the spider to have babies that had babies (and so on), there would be no environmental pressure, and hence no evolution (although there would be aimless "drift" of DNA).
Think about the common roach, which has largely remained unchanged.
Just from looks, I bet this bit of nastiness is pretty close to the current models, so the DNA would not be terribly different.
"Aimless drift" of DNA is also evolution.
And contrary to your claim, if we were convinced that the spider died 20Mya, and that DNA was not contaminated etc., and the DNA were the same as some modern species, some fundamental assumption of the theory of evolution would be proven wrong.