Snape's part in death of Sirius

[nkafkafi]

No: HPFGUIDX 107782

Potioncat:
Where does the 2 hours come from? Do we know how much time passed
between Snape discovering Harry was gone and his notifying the Order?

Neri:
The 2 hours came from Pippin's estimation. Lets check. We know for 
sure that Snape checked the first time with HQ after leaving 
Umbridge's office. It seems that his means of communication, whatever 
it was, was very fast or instantaneous, because he sent a question 
and got an answer that Sirius was indeed in HQ. We know that when 
he "grew worried" he contacted HQ again. So the time it took him to 
become worried was:

The time Harry and Co. spent in the forest +
The time it took them to fly to London +
The time it took them to find the prophecy +
The time they fought the DEs –
2 x The time it took Snape to contact HQ –
The time it took the Order members to get to the DoM once they got 
the message from Snape.

In my estimation it would be considerably more than 2 hrs.

Double-check: It is also mentioned that twilight was falling when 
Harry and Co. took off to London from the forest, and when DD sent 
Harry to his office after the battle it was already dawn (Harry can 
see the first light on the horizon from DD's window). So you can also 
calculate it as:

The time Harry and Co. spent in the forest +
The length of the night in Scotland in June –
2 x The time it took Snape to contact HQ –
The time it took the Order members to get to the DoM –
The time since they got there until DD sent Harry to his office.

IMO should be at least 4 hrs, more likely 5. Moreover, most of it was 
dark time. Snape waited at least 3 hrs AFTER DARK before he "grew 
worried" about six students and a teacher in the Forbidden Forest. 
Not responsible at all.    

Potioncat:
I am assuming, whatever the time, that the kids had left the forest
by the time Snape is looking there. Do we know "when" he found out
that anyone had left the castle?

Neri:
It would be very strange if he hadn't noticed for several hours that 
all the Inquisitorial Squad was hexed. At the very least someone must 
have noticed it (it must have been quite a BANG) and alerted him, as 
he was one of the senior teachers left in the school. How much time 
is it to question the Slytherins and find out what happened?
  
Potioncat:
This could be a fun, meaty thread to debate, but I do have one
major question. With all the errors we've made with Mark Evans and
the HBP: Is it your thought that JKR is showing us that Snape
delayed looking for Harry?

Neri:
Do you mean that it wasn't her intension and she didn't do the math 
of the timeline from everybody's point of view? Could be, but IMO 
this fits well with the pattern of Snape's carelessness and 
irresponsibility regarding Harry during the Occlumency lessons and 
especially after he stopped the lessons.

Neri