While RCI doesn't have that feature, Celebrity does. Try to find a Celebrity ship which home ports where you will end the TA around the date you need it. Look on the Celebrity version of Air2sea.
Rarely if ever does RCI announce their reasoning behind some of their decisions. It makes total sense that they would NOT advertise that they have a sizable part of the crew quarantined with COVID.
Having an assigned cabin doesn't isolate you for being bumped. All of the GTYs being bumped very likely had an assignment but it was just easier to bump them, but if there weren't enough of them, folks with assigned cabins would have been bumped as well. Actually, we don't know that anyone with an assigned cabin wasn't bumped or moved.
We don't know the entire story of why the ship ended up with fewer cabins that needed - sometimes RCI takes entire blocks of cabin out of service at the last minute for various reasons (like maintenance, etc).
As long as you switch a booking to a new ship/sailing it doesn't matter if you'd had that sailing booked under another booking. The RCI system even allows booking the same ship twice.
The Surf option (lower speed) was discontinued starting about a year ago and was only available on board toward the end of availability. As D you get a discount equivalent to the 24 hour package.
When they login and attempt to buy the 24 hour package, there should be a zero-cost option to do so. The Voom staff should be able to activate the package if the kids are under the age limit to have their own accounts.