Is This A Fake GB 1658 Cromwell Portrait Shilling?

I can't see anything obviously wrong with it apart from the "too good to be true" concern. Unfortunately the ungodly are able to produce extremely convincing copies of all coins these days, and when the potential reward is very high (as here) they pull out all the stops.
As well as the info you have requested, I would examine the auction/seller info as much as possible. If it is an auction house, their reputation, particularly when dealing with coins. Do they routinely have high value coins and collections through their hands? Do they have numismatically qualified experts on their team?
If a private seller online - check not just his feedback, but what he has sold in the past and what people have said about any high value items. What else does he have for sale? If a lot of other one-off high rarity coins, where has he got them from?
Another thought is to have a look at the source for many of the fakes - online websites such as *Site prevents me from saying which one!* - to see if they have similar items for sale at the moment and how they compare.
I am probably teaching my grandmother to suck eggs, but that is what I would do before risking a large sum on such a thing!

The dots on the scarve and the crosses in the shield look wrong, there appears to exactly even strike and almost no wear, yet its dull as dishwater.

Beware, I would not buy it without a certificate.

Despite it being kosher looking, my gut tells me this is a fake, a good one - but a modern fake that is likely Chinese!

Item was not sold, and has not made a re-appearance.