He should be able to deduce the nature of the dementors from Harry's invisibility cloak being able to hide the wearer from them (which should be readily apparent from Harry's summary of events and the fact that Bella was wearing it). Other things like what you point out or Harry's parselmouth description as life-eaters could help, but the cloak thing by itself should be quite sufficient given Quirrel's previously displayed deductive abilities. I don't think he will be able to cast a true patronus, though.
The fact that the true invisibility cloak effectively hides one from dementors doesn't necessarily mean that dementors personify death, only that whatever mechanism they use to detect people is also blocked by it. The cloak is said to be able to hide one from the gaze of death itself, but that doesn't mean that if the cloak hides you from something's gaze, that thing must be death. The hint Harry used doesn't really carry in the opposite direction.
The life-eaters part does sound like a clue though.
Would a person have to think about universally conquering death to cast a true patronus, or is it enough to imagine personally conquering it? If all it requires is the latter, I can certainly see Quirrelmort managing it.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.