1) Who rolls the last threat die on the last space? Does it have to be the Ring- bearer's turn to roll to get to the Cracks of Doom?
2) Does the descent into darkness occur in Mordor? If not, then what is the purpose of collecting life Tokens?
#1: You mean space #60 (and not Mt. Doom)? Active player rolls. Normally, I try to use the Ring from space 57, you have a 50-50 shot of moving 3 spaces (blank, 1 corruption and Eye) which allows you to skip both of the rolls in the final three spaces. Doesn't matter whose turn it is when Mordor's Main Activity line ends. At Mt. Doom, if the Ring-bearer makes it there, there is no Ring-bearer, instead the Active player rolls and if alive, win, if he fails, the next player becomes Active and rolls. Continue until you win or lose.
#2: Life tokens on the Mordor board are only used for Events 3 and 4 (at least in the base game).
If you already haven't give the below link a readthrough (or two or three):