OR, once they stop him, why can't he go back and try the same thing again, and again, and again, endlessly?
They did answer this one. You can't go back to the same place twice because if you run into yourself Baaaam! and the shock waves would have catastrophic effects all up and down the timeline.
Why doesn't the antagonist just go back in time and sabotage the Lifeboat so they can't follow him?
He axtually tried this once. The bad guys lutred the good guys back to the French and Indian War and attenpted to blow the lifeboat up. THe good guys got all but one oirce of explosive off and hade to jury rig a repair to get back to the present, The Techie managed to hobble together a crude capacitor and yes there was the mandatory joking mention of Flux capacitors.
Question here is why did bad guys use lots of little explosives instead of one big one nut we know the answer.
OR, once they stop him, why can't he go back and try the same thing again, and again, and again, endlessly?
The plan for stopping him is to kill him.
Guess the question here is why has the Delta Commando failed to do so on 10 different occasions.