Thanks for all that Nick ..

after reading those links, would it be true to say the main difference between your view and the so-called 'general consensus', is the lesser significance you give to the hydrological cycle? In your own web page your argument is based on the greater volatility of CO2, to quote:

Water ice is relatively refractory. Some of it melts or vaporises in the impact, but most is blasted out as unmelted grains. The CO2, on the other hand, is very volatile. It readily forms a gas with much lower energy requirements. Therefore, every time a large impact crater was formed on early Mars, a large volume of CO2 gas was produced which temporarily formed a huge cloud - perhaps even a planet-wide atmosphere in the case of basin-scale impact.

Well, that makes a lot of sense and your 'atmospheric layering' scenario (where the gaseous CO2 ejecta eventually settles down over the rubble) seems to answer your intial points, which I quote for anyone else reading this:

If the layers are sedimentary, then it implies an active early sedimentary history for Mars, involving a far stronger hydrological cycle than we have evidence for. If early Mars were this active, then all the early craters should have been wiped away, yet they are largely unaffected. In addition, layering exists within many unbroken craters, without sign of an entry or exit channel. It is as if the layers had fallen from the skies.

If the layers are lavas, then it implies a much greater early volcanic history than we see evidence for. Again, many of the large early craters should have been destroyed or buried, and we should see evidence for many more volcanic vents and fissures than we do in fact see. In one way, the volcanic model can explain the layers in enclosed craters. Massive volcanic eruptions could have formed ash clouds that rained down into the craters, yet we don't see the vents.

...

Why don't we see similar layers on the Moon? There the impact layers are more jumbled. What is different about Mars?

Anyway, my question is again, that the other links you mention (part of the 'consensus' I guess) don't mention the CO2 scenario at all, only the hydrological. For instance:
With respect to early Mars, two things are undoubtedly true: (1) there were numerous impacts by objects larger than 10 km in diameter, and (2) these impacts would have dramatic climate effects ... In these simulations, an initial warm and wet atmosphere, like that which would immediately follow an impact, is allowed to evolve back to a steady state. It is shown that even for relatively small impactors (D < 50 km), substantial localized rainfall can occur.

And also (from your google results): Maybe, argue a team of atmospheric scientists, it rained buckets on the young Mars following every major impact — then the climate reverted to frigid, dry conditions in between. At a meeting of planetary scientists this week, Anthony Colaprete (NASA/Ames Research Center) detailed how planetwide downpours could be triggered by the collision of an asteroid or comet at least 10 kilometers across. The impact’s energy would quickly vaporize all the ice buried in the target zone. It would also choke the entire atmosphere with white-hot ejecta that would heat the surface to at least 1,000° Kelvin, vaporizing even more buried ice. As the water-saturated atmosphere cooled, Colaprete argues, rain — and lots of it — would be inevitable. Some regional downpours might dump more than 2 meters of water over several weeks, while other areas would hardly see a drop. Within a few months the atmosphere would return to normal, ending the rains until the next impact occurred. So Colaprete at least, seems to be saying that the impact energies would be so high that water ice itself would be vaporized and blanket the planet in a similar way which you have described for the CO2. - Even though in the PDF paper you linked to, he admits the early sun was 75 percent fainter than today's sun (at Mars). I guess it comes down to impact energies, and the volatilities of H2O and CO2 under the temperature and pressure regimes of early Mars? ... And I'm sure, other factors as well of which I am unaware ...