I've posted a lot of OB questions that Chuck has patiently answered, and his answer has generally been: terrain objects (roads, trails, pavement, water, etc) make bad OB lines due to exactly the kind of vagueness you described. A tournament should have a clearly marked line in place to define OB properly, rather than rely on topography. But I realize you asked about casual play.
In casual play we usually say that if it's surrounded by OB then you're OB, otherwise you're not. Disc 99% on pavement, but 1% on dirt = not OB. Disc in water, but also sitting on mud that is arguably muddy shore or arguably wet streambed = not OB. Basically, we assume you aren't OB unless it's obvious. But again, that's just casual rules.
Shot 1 = not OB
Shot 2 = I'm not sure what you were describing. It doesn't matter that you crossed the road and came back (unless you never crossed back IB and landed OB, then it would determine the location of your lie). It matters where you land.