Not sure about 3, but for the DSC, if you’ve prepaid, you do not pay the difference. We did both Upsell and won a bid into Haven and both times we did not have to pay the additional amount. The bids are doubled so yes you would pay double the bid amount to upgrade. 🙂
They also do that on the off chance someone has to cancel last minute. I’m sure it doesn’t happen often but occasionally it does so they probably want to have options to fill the cabins if it does.