Hmmmmmm.

Either Tempus was smarter than I gave him credit for, so he DID fix it so they can't change events other than the car accident.

OR - Time travel is a lot more complicated than Tempus or Lois and Clark realize. (Don't forget, Tempus USES time travel, etc. but it was Wells that invented the machine.) So in this case, ANYTHING they change will create a new reality with a new Lois and Clark.
Therefore, if they go back and recreate the car accident, they will see a new Lois and Clark at the end, rather than get their lives back.

So I guess, Tempus really would win. Divorce and remarriage laws, anyone?
I still don't see how they will able to survive without their boys, though, even if they do have OTHER KIDS.