No, you won't have any issues with getting the CC now. Marriott does use a rolling 12-month calendar for achieving a higher elite status.

Also you might want to check which Megabonus promo offer you have. I do believe there is one version that has double nights. If you have that one, you can rack up even more nights this year (and they will count for 12 months after you had the stay.) Marriott does have a "Roll-over nights" promo going on. This means that any nights beyond the elite level you achieve this year will count toward your elite status for all of next year (rather than for just 12 months beyond when you achieve them.) But considering that we are in the last quarter of the year and you want to achieve Plat quickly, I don't think the Roll-over nights promo will help you.