If you can be sure to have the FLR(M) application on the desk of The Home Office BEFORE your visa expiry then you'll be OK
Your immigration status will remain in place and fully legal whilst your application is reviewed. So put those worries out of your mind.
You'll only be granted an extension to the fiancee visa if you evidence GOOD reasons. There really needs to be a good reason why you did not already get married.
As everyone has said get that application done and dusted NOW and fully ready to submit in plenty of time.
You both need to develop an urgent strategy to get married and submit the FLR(M) NOW
Just out of curiosity why have you taken such a huge risk and left it so very late?