Balcony cabins on Epic are tiny. You probably will not be happy with a single cabin in your situation.
BTW short answer to the question is that if a cabin says it sleeps four as long as one is 21 or older the other ages don’t matter.
We’ve done the 26 Glaciers tour. Very nice, but I would book directly and not through the ship as the price is about double.
BTW there is nothing in Whittier, and it is too far and difficult to go anywhere else.
It doesn’t matter. When the bus gets you to the port you will go right to the check in line. They don’t pay any attention to time. You will probably get to the ship around 12:30.