be hard pressed to call the road much more than a 4wd jeep road by american standards. many places aren't even wide enough to fit two passing vehicles. average speed is maybe 8 mph. we knew people who took jeeps to cut out a day of walking, and we literally walked into tatopani 2 hours later than them.
but even off the roads, you are still walking miles on wide trails, clogged with pack trains and animal crap. i mean, this place isn't wilderness and hasn't been for centuries. so i wouldn't think much of the road. it's very easy to avoid if you want, and it's not really all that busy.
from the east coast of the US, there are direct flights to delhi. but flying through delhi can be a real pain, since often you are required to have a transit visa if you check bags. if you just carry on, it's fine. although it seems to be randomly enforced.
i flew salt lake – chicago – delhi – kathmandu. ran around $1400. from JFK, i'd think you can find $1100-$1300 with ease. with patience, you can get from the states to delhi for under $800. flights from delhi to KTM are $130 once here.
overland travel from india sounds miserable. especially if you are trying to use your time wisely. it would take a lot of time, plus cost you an indian visa. there may be some trains, but it would be all buses in nepal for sure, and i'm sure jeeps for a significant portion of the india leg. unless you are visiting long enough to go to india too, i'd avoid it all costs.