I have read that at least one of these bugs is already known.. I do not know if there are any fixes to them. I am on Xbox 360 with the most recent update.

The first bug is the NCR quest where you "Take back the town of Nelson". I cannot complete this quest. This is probably because I did a previous quest where a nearby Ranger asked me to Kill the hostage soldiers being held there. Instead I ran in killed all the Legion guards and actually set the soldiers free. I thought it was pretty sweet and the Ranger actually commented on my effort in dialogue. However, now in the "Taking Back Nelson" quest there are no more Legion guards for the NCR to attack... so my soldiers just kinda stand there...

The second bug is the NCR ambassador in New Vegas disappeared after I got the Boomers to support me in the upcoming Hoover Dam battle. I completed the quest, but it seems like I cant get any further with NCR now.

Are you SURE the ambassador has disappeared.Or is he on the opposite side on the building fast asleep in his bed?