(1). What did you think about my argument here? That Quirrel couldn't think of a plan that started by teaching his most powerful and secret magic to a minion.

(3). Maybe the backlash is proportional to the strength of one's magic, so in Azkaban Quirrel was affected a lot more than Harry, and baby Harry wasn't affected at all. That matches the fact that Quirrel's new strategy for surviving such moments is to stop using magic, throw away his wand and abandon the very shape of a wizard.

(5). If he tortured or threatened Snape, he couldn't trust anything Snape might say. A perfect Occlumens is perfectly prepared to assume a false identity that will confess false information.

(6). Back then, Quirrel wanted to strengthen Harry politically and eventually help him rule Britain. If he killed Lucius, Draco would blame Dumbledore, be very angry and afraid, and probably leave Hogwarts. If he caused Lucius to be disgraced, he might succeed in making Draco believe that Lucius had indeed killed Hermione, and then Draco would come back to Hogwarts and become even closer friends with Harry.