With all of the initial media attention and stories on this case, why wouldn't at least 1 bone have been found by now?
Not necessarily. Depending on where someone entered the woods, they could wander for miles--a lot of that area is National Forest land. Even if it was just a few yards, hidden from the road, wildlife would scatter bones. Without knowing where to look, it's entirely possible no trace of a person would ever be found, or might be found years later by hikers or hunters.
If Maura decided to leave the accident on foot, which seems very plausible since she was, by many accounts, troubled about something, probably worried about getting in trouble as she had a couple of recent incidents, and possibly had been drinking, if she continued in the direction she was going and ran/jogged, she could have been at least a mile away by the time police arrived at the accident scene. If she ran in the road, tracks might not have been evident if the snow was covering them, depending on when the road had been plowed and how deep fresh snow was before and after she left.
Just a bit of speculation here, but I'm guessing LE spent at least a few minutes searching for her closer to the accident scene, at surrounding houses, etc. That maybe buys her another mile. Every mile further east she gets, the more remote the area. If she hid when she saw headlights, even just a bit off the road or in a driveway, she'd be hard to spot in the dark, and if she ran a little further each time the car passed and was out of sight, she could have gotten a few miles up the road. If she had gone into the woods that far away, even if someone had seen footprints, would they connect them to Maura if they were 4-5 miles away? Prints may have filled with snow by morning and appears older than they were.
I think if she was taken by someone, it's more likely that that someone saw her when she stopped (I think it was at a store in Woodsville or North Haverhill, IIRC?), did something to her car while she was in the store that caused it to break down/go off the road, and followed her. Or, that person followed her from Amherst, if you want to go for even more sinister. Perhaps that's why she was running in the first place. I still think exposure is the most likely, followed by suicide, but this seems more likely to me than a chance meeting on a remote road on a snowy night when most locals would be in for the night.