very little life can survive being frozen
On the contrary, and Samantha Wright please correct me if I'm wrong, but I'd think a whole big hunking lot of single-cellular life can in fact survive being frozen. I mean, come on, human fucking sperm even does. Never mind that frozen life is well, frozen. While the DNA repair mechanisms are dormant, so are the copying mechanisms. Bacteria can live quite deep within porous rocks. I'm not exactly sure if it's really necessary for ejecta to be always heated up to sterilization. Now I'm not saying that this little life-from-Mars theory has got any legs to stand on just yet, but your arguments don't really do much to discount it, I don't think.