First of all, whether you are rejected or not, will not impact your WP status. The only thing that would do that is if you got pregnant. (Hopefully, that isn't an option!)
But you do need to apply to MOM to get approval to marry, that's a fact. If you marry in spite of not getting approval, then your WP would probably not be renewed. Additionally, there is a chance that, if that happened, you might also be refused an LTVP based on your marriage, even though your wife has sufficient income to act as your sponsor.
My suggestion? Go ahead and apply. You've been here for 22 years and you are Chinese so for all intents & purposes you are already Singaporean (or probably think like one) Actually, your English is better than 95% of Singaporeans!
Even if you wait to see if you pass the exams in December, AND you manage to change jobs and get an S pass, you STILL have to get permission from MOM as the ruling says "If you have EVER been on a WP" which means currently or at some point in the past. (I can vouch for the authenticity of that from my own extended family) So, may as well go and try. At least that way, you can start making alternative plans instead of fretting too much about variables. As much as they want Chinese babies, I doubt you will have any problems. Let us know how it goes.