wavy appearance of roof between trusses
I think I would tend to believe the roofers on this one.
Any more opinions?
If you can get at the underside of the roof from the attic you can install 2x4 blocking to push the plywood up where it is dipping to help straighten it out.
If you're not in the snow-belt, you can probably get away with not resheathing until the next reroofing job. However, I suggest you periodically check on your sheathing from the attic to make sure no further deterioration is happening.
Best of luck
I like Glenn Good's suggestion about the 2x4 blocking. Obviously you don't want to reroof after just having it done. Try the 2x4's in one area and see if it makes it look better. It might take a while, but it's a lot cheaper.
Keep this in mind when you want to get up on your roof. If the posts are correct I don't think I would walk up there. Maybe take a wood plank up with you and lay parallel to the peak. Walking on the planks will keep you from falling through.
IMO, they should have noticed the lack of structure, notified you, and given the option for you to agree to a change order to replace the plywood.