1- Harry could conclude Quirrell made the Pioneer Plaque into a horcrux without coming to the conclusion that he's Voldemort. That Quirrell has made at least one horcrux is evidence towards him being Voldemort, but not all people who've made horcruxes are Voldemort.

2- This would probably convince Dumbledore that Quirrell is the top candidate for being Voldemort, but I think Dumbledore would need to follow it up with more investigation to convince Harry; I don't think Dumbledore has much more evidence to bring to the table if Harry lets this spill that he hasn't told Harry already.

3- Suspecting Quirrell as the plotter hasn't led Harry to suspect Quirrell as Voldemort yet, and I don't really see how hearing David Monroe's explanation of his backstory would lead Harry to conclude he was toying with Magical Britain for fun; he could simply conclude he's a largely amoral person who tried being good for the reward, but concluded that it wasn't rewarding.