I like my plan better. I live in a secure apartment so he wouldn't be able to get in if he missed his window without informing me of a delay.
I haven't quite worked out what I would tell the girls though. That there is the hole in my plan.
Unfortunately it is both his failure and a reflection of their importance to him. Its not your job to clean this up mess for him. I know it goes against your protective instinct as much it does mine but sugar coating this for them won't be to their benefit in the long run.
They need to know they are important, absolutely. They need to know that the failure is his, absolutely. You need to try to do that without minimising the shitty things he is doing.
If it is a one off - so be it, people are late. They will learn that it happens sometimes. If it is regularly then.... I'd be inclined to not agree to any further changes without specifying that if he is more than 15m late without notice he is to consider the additional day deal off the table.
I'm a newbie at this and my girls are very little. I'll be reading the (perhaps more reasonable/viable) responses here.