I probably should think longer before I respond, but my girls are playing happily in the tub, Sebastian's asleep, and the stars are aligned so I decided to take advantage of the moment, and respond as soon as I read your thoughts.
The first thing that popped into my head when I read, "Our reasons for sinning---and for choosing not to sin---are often motivated by many factors, and while that doesn't make the sin less wrong, it does make ME more hesitant to judge others" was a talk I heard in EFY. It's been a long time so I don't really remember all of them, but the speaker was saying there's different motivations for choosing not to sin, some better than others. I think it went something like this.
1. Fear of what others think or will say (worst reason)
2. Fear of God, hell, etc.
3. I forgot
4. Love for others (our family, friends, etc.)
5. LOVE of God and wanting to please him (best reason)
Anyway, I think there are a lot of ambiguities in life. I was just listening to NPR yesterday and there was a guy on who wrote a book, Bargaining with the Devil: When to Negotiate, When to Fight. Here's a link to the interview. It talked about many of the issues of what you discussed. And then again on NPR, I only caught a snippet of it, was a discussion with this guy about whether it's better (in his hypothetical example) to push one guy off a bridge to his death in order to save 5 people that soon will be crossing the bridge. His real life example of it was a when a group of 4 Special Operations soldiers were deployed into some hills along the Afghanistan/Pakistan border with the knowledge that some top Taliban leaders were in this town nearby and they were supposed to take them out. By random happenstance some Afghan shepards with a bunch of sheep came up on them and they had to decide whether to kill these innocent Afghan shepards or risk that they'd go alert the Taliban and ruin their mission. Well the four guys voted and decided they did not have the moral justification to kill the shepards and they had no way to detain them, so they let them go, and within an hour they were surrounded by Taliban and only one escaped alive and a rescue helicopter was shot down too so another 11 guys died as well. The soldier that escaped said he wished he would have chosen to kill the two shepards instead. Anyway, definitely a hard call, and one that I'm glad I'll never face.
I think that Adam and Eve gained the knowledge of opposites (joy vs misery, right vs. wrong, pleasure vs. pain) but that doesn't mean that there isn't things between those. For instance, very rarely do most people feel absolutely miserable or absolutely joyful. And while I realize I'm on a slippery slope here, I think the same applies to right and wrong. Something might be right for one person and wrong for another. For instance, homeschooling, or having ten kids. Or what about that lady in the Bible who drove a stake through the bad guy's head (sorry I don't recall the names and I'm too lazy to look it up) after being all nice to him and letting him sleep in her tent. Right or wrong? I don't think the Bible is very clear on that one. (But then again maybe I'm not remembering correctly). Anyway, point being is I agree that only God can know someone's heart and so He's the only one able to judge with absolute understanding and the rest of us just need to rely on the Spirit to make our own decisions and not judge other's decisions. Easier said then done, eh?