I agree 100% it is super hard to measure a boat across the beam...wonder how most do it. I know i use a computer model....that is how manufacturers now determine dimensions. Most actually use the outter flange of the fiberglass of the hull where the hull to deck joint meets, few actually include the bow rail, some are starting to as of lately. Not sure which Grady does, and there is no such thing as a 100% identical boat, depending on when it was built and how well the hull and deck fit...ie it might have needed clamps to bring it in to fasten them correctly, one boat to another may have slight variations, on a boat that size maybe of upwards of an inch is possible depending on how long the hull and deck are allowed to cure before being pulled from the mold, as well as how much structure is put in before it is pulled, one with less structure will be allowed to change shape more.
Only real way to measure properly is to get 2 big sticks...2 tape measures and do it the old fashion way and get the same measurement on each tape and make sure it's squared off. I know almost no marina's will do that so their measurement is more of a guestimate, nobody can perfectly eyeball a beam measurement when the hull has crown and in your instance...a windshield where the max beam is.
Honestly I've never heard of any marina's worried about beam measurements, they usually take the word of the manufacturer, it is not worth the hassle to deal with, they typically want to know overall length so they can charge you as much as possible and make as much as they can, beam is irrelevant as long as it fits between the slips poles. I know they might have rules...but in reality, will 2" matter if the boat fits between the poles? Answer is NO
I would try and have Grady put in writting that the boat has a 8' max beam, in court this should help, and if the club heard it might go that far and you have documentation they might settle before it gets that far. Also, question how they took the measurement...ask how they squared up the edges to know it was aligned with the rub rail...did 1 or 2 guys do it? It is a 2 guy job to take slack out...maybe even a 3 guy job. did they use a measurement off the transom along the sheer line to identify a spot where the max beam is to make sure they measured on a straight line across the beam or did they measure on an angle, a slight angle could add inched easily, add that to not having the tape squared, and there are your 2" Also, a tape measure is not right for this job, it sags....you need a carpenter's ruler honestly. Tape measure's are absolutely HORRIBLE for measuring off, the right way is using a carpenter's ruler. We rarely use a tape measure to measure anything. For any precise measurement's, it is a carpenter's rule. Bet this is a little food for thought...hope some of this help's...I'm sure you can catch them up somewhere here....I'd suggect measuring it yourself these ways though as well just to know what they might find if they were to actually go do it the right way now.