MGR is a great trader....IMNSHO
I think it makes a really nice asset. As for the FC, I've been able to get a good trade at least once (the only time I tried)
I found this little excerpt about FC rules, but can't vouch for it's truth. MBP (Ocean Point) is the only 3br AFAIK, and I'm not sure if you can do a 3br-3br FC trade. I will slay this: when I log onto the booking website (too early to do FC booking now...) it does allow me to select "Full Villa" and "Ocean Point" (only OP with full villa selected) I am led to believe that you can do a 3br to 3br booking. Other Tuggers have reported it's possible. It's also possible to lock-off, book a 2br and a studio and then request that they be combined (via room control or Marriott) but good luck with that.
As to the actual success of being able to book a 3br MBP unit with FC at 6 months out..... Who knows? Start a thread under Marriott and see who might respond. I think
@GrayFal has pretty extensive experience with MGR exchanges to MBP.
Hope this helps. Just remember: can't book till 6mos, like for like (size and season.