I don't know if I can really advise you not to bring the ATV, it just doesn't make sense where we hunt in particular. We're a fair ways in from the trailhead close to the wilderness boundary and it just doesn't make sense there. However there are a few spots where I could see a guy using one. I'd just hate to tell you, you'll definitely not need the ATV and then wish you had one coming all the way from NV.
Regardless, I think the setup and park and hike in a couple mile scenario is not a bad way to hunt especially for a first trip till you can gauge the particular area you'll set up in.
I wouldn't worry about the bears too much. I've been in that country quite a bit and seen a black bear only once, and that was in the Spring. I have seen pictures of Grizz as far South as we hunt, but I've never actually laid eyes on one nor cut a track. To my point, it will be my first time archery huting in there this Fall and I will not be packing a sidearm. On the far North side of 98 maybe, but not the Southern side.
I'll try and post some pictures here if I can figure out how.