Wouldn't be surprised if A strain affords some protection against C - hopefully scientists will look into this.
I believe scientists have isolated various strains, but these are very, very similar, and there’s no evidence (at least that I’ve seen) that any of these strains have dramatically impacts on mortality. If there was a “mild” Covid precursor to the current pandemic, it seems to have been thoroughly supplanted by the current more aggressive strains.
The high death rates in Bergamo vs London could be explained by many things, age of population and/or initial strain (if we were swept by A strain first, for example, whereas Bergamo's predominant initial strain could have been C.
Unless Bergamo has a concentration of elderly and those with underlying conditions that is more than 3 times London’s and at least double any other Italian region, and massively more than those in the south, this hypothesis doesn’t stand up to scrutiny.
As for “strain C” theory developing in Bergamo, for this to have worked, AND for strain A to provide some immunity from strain C, Bergamo would have had to have largely been isolated from strain A, with that isolation reducing with distance from Bergamo.... and for strain C to have emerged within Bergamo in February and then spread out accordingly, affecting areas in proportion to the number of strain A cases.
For this to happen naturally, it would be a bit like putting dye into a bucket of water and for a clear patch to remain while other parts became coloured. Technically this is possible, but the odds against it happening are trillions-to-one or more.