Not a creationists but this argument is just an attempt to get around the start of life thing. It doesn't matter if life came here on a comet, how did it start "spontaneously" on some other planet if it can't do it here, and how in the hell did it leap off a planet onto a comet? This argument is a cop out! So, no, you can't have one without the other.
Not at all. It is a quite valid theory. One can decide whether a higher power created that life or whether it arose of its own accord - just like we do and debate about now.