The price guarantee is what it is - how far out is your cruise? I assume less than 90 days?
I am sorry, but I feel you are beating a dead horse here. Like I said, an airline or hotel would not lower the price for you, cruise pricing is based on a strict set of rules - deposits, final payment, refunds, etc. They don't change them just because you ask.
I think you were lucky to get the onboard credit and you have done all you can, please don't let $25 difference ruin your whole cruise experience.