I love these questions and this kind of planning. My short answer is no, you can't reliably get there. You may get lucky, but you can't plan on it. My longer answer is this:
Well, you're traveling in the right direction, because the TTC, which is where you get off the bus for the MK, is also a hub. So you don't need to transfer. But you will need to get from the TTC to the MK by monorail or ferry. I'm assuming the official MGM park opening time is 9:00 on the day you're thinking of doing this. If that's the case, I don't think you're going to make it; at a minimum, it's going to be very very close.
Even with no lines, TOT and RRC take a while, because of the preshows. If the turnstiles open for early entry at 7:50 or earlier, you may have a shot. I think you'd need to get in, ride, and get out to the bus waiting area by about 8:40 at the absolute latest.
I believe at this time of the year, the busses go about every 20 minutes; sometimes more like 25. That makes leaving MGM at 9:05 your worst case scenario. 20 minutes driving time, plus ferry or monorail, and you'll do very well to get to inside the MK turnstiles by 9:45. Where is your PS? Even Crystal Palace is a 7 minute walk or so from the turnstiles.
Two caveats. First, if MK's official opening time is 9:00 that day, you're in trouble. You'll hit the TTC at the same time that it's being innundated with parkers who need to take the monorail or the ferry. If you're talking about doing this on the Saturday EMH for MGM, forget about it. If the Tuesday, you might have a chance. Second, the above assumes you'll wait the full 20 minutes for the bus from MGM. If there's one waiting there ready to go, you're probably safe, but you can't count on that. However, I'm not sure that 20 minutes really is realistic. While that is generally the interval between buses, I wouldn't think that there would be too many buses leaving MGM before EMH is even over. Maybe others can help on this, but there may not even be bus service FROM MGM at least until official park opening.