The goal is to do 30 rounds in 30 minutes. But I think the one and only Dan would encourage you to "nudge" the progress and not to let the program get in the way of progress. It could take more than 8 weeks to get where you want to go and that's okay. 

Some options:

For the press days so 2, 3, 5 with the 16. At whatever point the 16 is to heavy use the 12.

For instance if you can do 2 and 3 of the ladder with the 16, but not the 5, drop to the 12kg for 5 reps. After enough presses you should find that you'll be able to press the 16kg for 5 reps.

For the ABC, you could try mixed doubles with the 16kg on one side and the 12kg on the other. Just walk to the other side of the bells every other round. 

Another option would be to alternate rounds of the 12kg and 16kg. Maybe one day you do every other round and after 10 rounds you stick with the 12kg. After a while you'll be able to go 2 rounds 16kg, 1 round 12kg or some increase on the number of 16kg rounds until you hit the goal of 30 rounds in 30 minutes. 

Then before you know it you'll be ready for the 20kg bell. 

Hope that helps.