Im new to this and have been reading the last posts on the other topics and stuff for the last couple of days, and I'm not really old enough to be able to help in any way, but just here to give some ideas and other things that might help.
So I have a few points which might make the job a bit easier, but maybe not (and they may have already been considered), so if they don't help then just disregard them
1. In the anime in the first episode i think it was, kirito explained that there was no actual 'pain' in SAO, but there was still the sensation of touch. I think this could make things a heck of a lot easier, as the 'NerveGear' wouldn't have to make you feel things in a really 'high quality' but only in the sense that something is there. this could be easily done by using a suit which sends electric impulses to a part near the where the character is 'hit', but i think it could be fairly similar by using a helmet, by sending impulses to the 'rough area' of were the character in game is being touched, and not having to be too precise about it.
2. There are a couple ways that the persons physical body could be put to 'sleep' while in the game. First off a paralysing agent could be used, as someone else has said before. But there are also other ways that this could be done. Im not entirely sure how this works, but electric signals in one direction can be cancelled out by sending an electric signal of the same magnitude in the opposite direction right? if that can happen, all the computer needs to do is scan the strength of the current (then it has the data for moving the in game character) and then send a signal through the same nerve to cancel it out. If that can't work then another way you could do it is by using the opposite muscle to negate movements, like when the bicep is flexed the tricep is flexed enough to cancel out the movement.
3. The technology we have now is good enough that we have photorealistic screens, so it won't be very long until these become commercially available, and these could be used like the VR we have now where there is just a screen in front of your eyes. If no one likes that, a man who was blind can now slightly 'see' through cameras on his head that transmit data to the brain. this isn't very high quality at all and from what i have read he needed surgery to get it. But high quality versions won't be very far off in the future either, and the input can be from the game instead of a camera.
4. Sound can just be done through headphones or something similar. easy peasy hahah
I have no clue about the smell or taste but i hope this may have helped someone?
I've read about how the main difficulty is reading and writing signals to the brain, but i have no clue on how this could happen because I'm not even out of high school yet hopefully this helps someone but thats all i have for now.