I agree that the likelihood of extant life on mars is low, but it is premature to junk this out of hand. A few points:

Ice mists settle on surfaces (I’m Canadian don’t argue with me on this one). What Mohlmann is talking about is not bulk liquid water, or sublimation, but water adsorbed to surfaces where he maintains it behaves as a 2D-liquid even at very low temperatures (Icarus 168 (2004) 318–323), This so called 'unfrozen water' is only a few molecular layers deep. On our scale this is not very helpful, but microbes are nanomachines and this is significant at their scale (the cell membrane after all is only two molecules deep). This doesn’t get around the problem that it is bloody cold, and the reaction rate will be ‘glacial’, but Mohlmann’s hypothetical organisms may be very slow. They have had a bloody long time to play in the sand. Time “means’ different things depending on the organism; from the time I was born to now, the well fed E coli in my gut could have undergone roughly 1.2 million cell divisions.

As you obviously know, ice does not does not preclude chemistry. It allows species and intermediates that are unstable in liquid water to accumulate. For example Park et al, (Angew. Chem. Int. Ed. 2001, 40, 1497) looked at proton transfer between hydroxonium and ammonia and found that an ice surface allows chemical species to be formed and kinetically stabilized, so that water, hydroxoium ions , ammonium and ammonia coexist. Chemistry on the surface of ice seems to depend on the movement of surface water molecules and their ability to hydrate ionized species.