I think beyond is thinking of the Sen. Booker situation a few years back when there was a special election. In that case Booker and the Sen. that Christie appointed split the slate...5 each. The reason why was Booker was sworn in in Nov. not Jan. Thus, it was special circumstances. However, they never submitted two slates for a total of twenty, it was ten total. Think about it. There are 535 MOCs that would screw with the class size since they must charge these appointees. Even, if 25% of the MOCs change hands, you are looking at @135 more plebes for that year. SAs cannot have by law more than 4400. Every cadet/mid that commissions will be there for 2 election cycles, and one Sen. Election cycle. That will equate to about 300 more appointees if they allowed the outgoing and incoming to both submit slates.
Otherwise listen to USNA, the slate is charged to the seat not the person. Even when Obama won back in 08, the slate he endorsed as a Senator was the one submitted,not Roland Burns.. I agree I can't recall any slate ever being delayed until after the swearing in. If they did that there would be upheaval every other year for your Congressional nomination. It would also be upheaval every fourth year when you have both a Sen. and your Congressmen. That would be possibly 2/3rds of you sources changing hands.