It's weird that you can drop a player whose game has already started/finished to add somebody else who hasn't played yet, immediately bypassing the waiver wire. I don't remember that being possible in prior leagues.
That's either a bug or a bad setting. Which JL is it?
Once a player's game has begun, they should be immediately locked. Dropping the player the day after is considered a Week 2 action and will not affect your Week 1 bench.
Allowed:
1. I have an open roster spot Sunday Night
2. I add Ted Ginn Jr (plays Monday) and can use him this week if I choose
Not Possible:
1. I had Charcandrick West on the bench thursday
2. I want to drop him for Ted Ginn Jr on Sunday
^ In the above example, one of these two things must happen:
1. Can drop as a week 2 action, but since you don't have the week 1 slot, no pickup
2. Can't drop until tuesday
I'm not sure which way Yahoo does it, but there is fundamentally no way to drop a guy after using him within the same week.
I would ask your commish right away.
EDIT: ah, Outsider is on top of it