re: Comets shower
"Maybe the constant showering of comets maintained a higher atmospheric pressure and higher temperatures, also on Mars."
No doubt comets did their bit of stirring things up at the surface, as well as great deliverers of water (among other things/carbon, amino acids, possibly).
If Mars had a magnetosphere then, it would definitely have mattered and water and life may have had a chance, but it seems that all chances stopped at around three billion years ago (or so).
IMHO, when Mars inner core ceased/cooled(no magnetosphere), so did the chance for life on that planet. Whatever survived must have evolved and to it's worst case scenario has become fossilised.