Question about Snape and Quirrell

[marephraim <htfulcher@comcast.net>]

No: HPFGUIDX 49628

Forgive if I've missed it in the archives (usually do), but several 
questions occurred to me concerning PS/SS. At the Holloween Ball, 
Quirrell lets in a Mountain Troll, races to the Great Hall and 
pretends to pass out from fear. Everyone panics, Dumbledore sends 
the students to their towers and the teachers are supposed to follow 
him to the dungeon, where the fainted Quirrell has indicated the 
troll to be. Snape is suspicious and sneaks away. 

Now this is what I don't quite find logical. Quirrell later tells 
Harry that after the troll incident Snape never again trusted him. 
Further, he says that snape was 'always suspicious' of him. (p. 210 
UK paperback, throughout)

I ask myself, Why would Snape be suspicious of him at all before 
this incident? Or was he suspicious of Quirrell from the time of the 
great feast when Harry's scar hurt him for the first time? (p. 94)
(The movie would 'clarify' this by having Snape glance suspiciously 
at Quirrell when the scar incident happens but Canon makes no such 
reference.)

Why would Snape break (unnoticed?) from the group of teachers going 
to the dungeons to look for the troll and go straight to the 3rd 
floor corridor? Did he find Quirrell with Fluffy or did he get there 
first? If the former, why didn't he go to Dumbledore straight away? 
If the latter, how did he meet up with Quirrell -- and McGonagall -- 
in toilet after Ron and Harry defeated the troll? (p. 127-131)

Further, why didn't anyone tend to the poor fainted Quirrell in the 
Great Hall (which would have prevented him going after the stone)? 
Did the students just clobber him as they stampeded to their 
dormitories?

Quirrell indicates his sorrow that the troll didn't beat Harry to 
death and that Snape only received a wound. Does this mean that the 
troll was also supposed to kill Harry? If so, how could Quirrell (or 
the troll) know that Harry and Ron would be going to fetch Hermione 
from the toilet? I ask this because he comments that for all he knew 
Harry might have seen Quirrell coming to look at what was guarding 
the stone (p. 210, referring to p. 99). 

All in all, to me at least, this whole sequence doesn't make sense. 
If Quirrell brought the troll in to get past Fluffy, why go to the 
Great Hall and announce that it was there at all? With the feast 
taking place, no one would have heard the Troll and Fluffy fighting 
it out so far away. 

Any comments or explanations to help me reckon this out?

MarEphraim