It ought to be editable, Semi... The Intros and endings are style Divisions, just like the Variations and Fills...
I'm afraid I'm not familiar with the Yamaha Way, but hopefully it's as easy as Roland. Use your Style Editor (or whatever Yamaha call it). Choose the correct Ending, and each Part ought to have a list edit. Go to the Part that has the sound that is too loud, find the last note(s) and turn all their
velocities (not their volumes - that'll turn the whole Part down) down. Might be a good time to make them longer, too, if you think it ends too abruptly.
Sorry I can't be more specific. Probably Ian is the guy to get the specifics from, he does this all the time...