No, they will not move people around to accommodate Mosaics. If sitting together is important to you, then pay the 1,500 points and be done with it. With a flight that full, the chances of 3 people sitting together based on a complimentary upgrade are small, but it's possible. Keep in mind that additional seats may still be sold for the flight, people may upgrade to EMS from regular seats, and that you may not be the only Mosaic on the flight angling for an upgrade to EMS.
If you don't want to spend the points, your best bet is to arrive as early as possible to maximize your chances of getting an EMS seat and ideally getting them adjacent to one another. Often what happens in these situations where the flight is close to full is that you end up with a bunch of random middle EMS seats, but it depends on a whole host of factors, many of which end up having to do with luck.