At the reaping if someone volunteers can the original (chosen from the glasses jar thing) tribute say no if they want to compete themselves, say a career was chosen and another career volunteered could the first career compete and say no to the other one?
As shown in Catching Fire the tribute cannot override the volunteer as Haymitch gets chosen then Peeta volunteers. Haymitch does nothing about this even though he agrees with Katniss to do whatever it takes to stop Peeta going into the Quarter Quell.
As for volunteers volunteering instead of each other; in Districts 1, 2 and 4 the fact they get the careers volunteering is mentioned. There will most likely be a system to do with whoever does it first.
But there is no canon evidence in the trilogy of books. Maybe an interview or something in the films though.
No, the reaped tribute can do nothing to veto volunteers, as seen in Catching Fire.
Before the Reaping, Katniss had talked to Haymitch about keeping Peeta out of the games if at all possible, and he told her that if Peeta was reaped, he would volunteer in his place, but that if he himself was reaped and Peeta volunteered, there would be nothing that he could do.