1 Yes, the HHN voucher can redeemed at guest services for an off-peak ticket that you can give to anyone. I've done that myself. It can NOT be upgraded to a peak ticket or frequent fear :-(
2 I use valet almost every day and have never had a problem, except at peak times it can take 15 minutes to get your car back.
3 I tip $1
4 valet entrance is across from the hub drop off entrance, next to mini golf; you enter the walkway to CityWalk just before the last moving sidewalk.