Harry without Hogwarts might have turned out to be like Ikari Shinji from Neon Genesis Evangelion. (Mind you, his aunt and uncle weren’t neglectful, Shinji just was antisocial, but the parallels are still there.)
No, I don’t think so. Being an orphan contributed in a small way to what Voldemort became, but he was born evil. Remember how the Gaunt family was described? [paraphrasing] “An ancient wizarding family with a streak of instability and violence, due to their habits of marrying their own cousins”. He was already genetically predisposed to be fucked up, but Voldemort has no redeeming qualities. I don’t believe his mother’s presence would have changed who he was.