I've also been thinking along these lines, anyone remember this part from the opening ceremony?

The young, thin, nervous man who Harry had first met in the Leaky Cauldron slowly made his way up to the podium, glancing fearfully around in all directions. Harry caught a glimpse of the back of his head, and it looked like Professor Quirrell might already be going bald, despite his seeming youth.

"Wonder what's wrong with him," whispered the older-looking student sitting next to Harry. Similar hushed comments were being exchanged elsewhere along the table.

Professor Quirrell made his way up to the podium and stood there, blinking. "Ah..." he said. "Ah..." Then his courage seemed to fail him utterly, and he stood there in silence, occasionally twitching.

"Oh, great," whispered the older student, "looks like another long year in Defence class -"

"Salutations, my young apprentices," Professor Quirrell said in a dry, confident tone.

It seems to imply that becoming the second victim of a Horcrux might not necessarily create a mishmash of personalities, but instead have them competing as separate (maybe "partially mixed"?) identities. This would also explain why Harry consider his "dark side" different from himself.

This lack of a definite personhood may be related to the answer that Quirrell gave when Harry asked him why he wasn't like the other children.

[Emphasis added]

I will say this much, Mr. Potter: You are already an Occlumens, and I think you will become a perfect Occlumens before long. Identity does not mean, to such as us, what it means to other people. Anyone we can imagine, we can be; and the true difference about you, Mr. Potter, is that you have an unusually good imagination. A playwright must contain his characters, he must be larger than them in order to enact them within his mind. To an actor or spy or politician, the limit of his own diameter is the limit of who he can pretend to be, the limit of which face he may wear as a mask. But for such as you and I, anyone we can imagine, we can be, in reality and not pretense. While you imagined yourself a child, Mr. Potter, you were a child. Yet there are other existences you could support, larger existences, if you wished. Why are you so free, and so great in your circumference, when other children your age are small and constrained? Why can you imagine and become selves more adult than a mere child of a playwright should be able to compose? That I do not know, and I must not say what I guess. But what you have, Mr. Potter, is freedom.

Perhaps he is a Horcrux transfer (as long speculated) but a failed one; introspecting about how different he is from his memories of 'himself', he would realize 'he' hadn't survived and all that was left was a weird mishmash of Monroe's personality and Voldemort's memories, and this was entirely worthless as immortality.

If Monroe was a hero, then Monroe's personality really doesn't fit with some of Quirrel's actions. But there are times when it seems like Quirrel is developing some affection for Harry, for instance the Christmas present.

I have long been wondering if there is a possibility of Harry redeeming Quirrelmort, and if there is actually any part of Quirrel still in there, it makes this idea a lot more plausible. I had thought that after the redemption, Harry would deduce that Quirrel is Voldemort. But I think there is not enough time left in the story for this to play out.