Should I leave the girl I have been with for 8 years, who has endured a lot for my sake, based on the words of a "fasiq" (transgressor/immoral person) about her past, knowing that I had heard such talk about her before I knew her, and that the girl claims what I heard is not legitimate proof according to Sharia for me to leave her?

The questioner must repent for the Sharia violations that occurred with the girl. What matters is the girl's current state, not her past. If she has repented and become righteous, then he should propose to her. He should not pay attention to what the man narrated about the girl's past. The default assumption is her innocence from what was said about her, coupled with the demand to conceal a Muslim's faults, especially concerning what they have repented from.