I agree, but using billm's point of view, the CP with imputed income and the NCP are off the hook for any S7 because the grandparents or new spouse paid for it. Using my example from earlier, a stay-at-home spouse could have her income imputed, but based on what I read here, it wouldn't matter because someone else paid the bill and both parents are off the hook.
I agree that a NCP shouldn't pay more than was actually paid for the S7 expense, nor should it be any higher than his/her determined portion, but if someone else from the CP's family is paying or helping out, then that shouldn't negate the NCP's S7 obligations. Who's to say that the Op's S7 contribution doesn't go back to the grandparents to help offset what they paid? Who's to say that they may allow their daughter to keep it as a gift?
In my opinion, the Op will go down a rabbit hole, if he refuses to contribute to the S7 that was paid for by the grandparents.