Ok. What I mean is when the Hellas impact occured parts of it may have gotten to Earth and brought contamination at the time. While this is too early for the dinosaurs ( I misspoke like Cheney) we don't know what level of life there was on Earth at the time. Also the figure of 3.6 billion years is just a vague estimate. I doubt many in the field would consider it outragous to say the impact might have happened 1 or 2 billion years ago, so the actual timing of contamination is unknown with respect to Earth life. If the contamination was extensive it may well be that the life forms on both worlds were reset and mingled and then evolved from there in a similar way. Then presumably Mars if it ever lived died off or at least had much less life later.
Nonetheless a larger impactor would be more likely to bring organic material, and there have only been a few of those. As to the igneous rock I just vaguely recall reading something on the subject. The point was that the kind of rocks likely to get here are from underground, and not the best ones to be looking for fossils in.
I'm not implying there is life there or that back contamination doesn't occur. What I am saying is that since we have found probably no life signs in meteors that implies either there was no life there or the rocks came from areas where no life could get into the meteor.
Either one seems to remove the idea of contamination somehow resetting the ecologies of the two planets so an MSR would be safe. If there was evidence of life in lots of meteors going through history I would say that it is likely to be safe. Since there isn't the other two possibilities apply. Either there's no life and nothing to worry about. Or there is life and it's not mingled into our own ecosystem. The third possibility I suppose is we don't have enough meteors to know for sure.
Since it does appear that smaller impacts are not bringing life from Mars then that leaves the possibility that only the larger impacts could. Since there aren't many large impacts it comes back to a major event like Hellas. If smaller craters are bringing material here then Hellas must have brought a much larger amount. If Hellas brought nothing I doubt any other impact did to any great degree.