Stop typing and do this.I really doubt Yamaha has been misprinting the manual for 6 - 7 years, plus, myself and several other riders have set the slack with Yamaha specs and no problems at all. A friend has ridden his for almost 20k miles since 2016 and NO PROBLEMS. Plus, measuring on centerstand WILL throw different readings, try it, I did.
Go adjust your chain to 15mm then sit on the bike. Now have someone check how far they can move the chain. Now have your girlfriend push down on the rear of the bike while the other someone tries to move the chain. There's your answer. Keep in mind your girlfriend came no where near how much farther the rear suspension will compress.