I have replaced mine 3 or 4 times over the last 20yrs. The plate holds the microwave while you hold it in place and screw in the top lag bolts. Your 75% is perhaps correct until the lag bolts are secured. Once the lag bolts are secured almost all the weight is on them. It’s almost as if the plate is there to help a single person install. How much you bust your back while getting it on the lip of the plate is irrelevant. A single screw for the plate into a stud is sufficient so long as it’s somewhere in the middle of the plate and not all the way to one end of it. Toggle bolts for the rest will suffice. Remember…measure twice drill once when drilling into the uppers for the lag bolts.