[LBraden]

to quote Chief Miles Edward O'Brien "I hate temporal mechanics"

If we go with the now-accepted view of the "infinite quantum reality" theory, we can argue that Bruce was only travelling alone one reality, were, lets argue that humanity went tits up in 1989 (if I recall correctly), the US started rebuilding in 1994, was a strong power again in 2050, and lets say ended up with regular space travel to and fro Mars in 2100.

However, what could have happened was that he also saw the problem with having "Prime Base" and went back in time to try and stop it, but he could only travel linearly, in other words he could not go back a further 10 yrs to tell himself to double check and stop it.

As for Krell, we could argue he too has the ability to time travel, or that he is using a project cryobed

However, this all falls under the purview of Dulmer and Luskly, not an engineer.