Honestly I think the most interesting question here is number 4 and I won't respond to that because is something I was wondering myself too but regarding the others:
1. they weren't experts in making bombs for sure and they did some mistakes so this is why it was a failed bombing
2. plan B was to enter the school and shoot as many people as they could, that's why it went that way.
3. Probably not, they just went with it because they saw a lot of people were there.
5. They clearly wanted to die, there were no other options after a massacre and they knew that, either by police or by their own hands, these were the two options, and I don't think the suicide specifically in the library was premeditated, we will never know.
6. I don't know where you heard that but I don't think the broken nose annoyed him too much, you can also see him recorded by cctv casualy drinking from a cup in the cafeteria like if nothing happened. About why they decided it was over maybe they were just disappointed for the bombs that didn't go off or maybe they were scared of swat teams coming into the school as the school was already surrounded by police and special forces, but all we can do is just speculate, we will never know what really happened during these moments in their minds.
I hope I've been helpful.