They wouldn't know that they were being deceived. I didn't know I was deceived when I was an atheist which is inherent in deception. You would only find out after your eyes are open to the existence of the God and the supernatural.When deception is on the table as a possibility for one, it's a possibility for all. If anyone was deceived by evil spirits, how would they know? Maybe you are being deceived.
Is it possible, rightly or wrongly, to think the evidence for God is not enough to warrant belief whilst not being deceived?
No to your second question. Your deceived until you believe. After that if you don't want anything to do with God, you reject him and go back to your former lifestyle you would be one of those persons spoken of in Romans 1. At least that is how I understand it.