Muttley - Nice, but I don't entirely buy any of those explanations. The F13 theory is certainly the most common one, & I think was favoured by my unrememberable source, but the key thing is that it is definitely a Victorianism. It doesn't go back very far at all, for a superstition.

Dropping salt bringing bad luck is a new one on me (throw it over your shoulder, well known). Whilst salt was sufficiently costly to have generated "below the salt" as a phrase, I can't see why seven years bad luck might ensue. It wasn't *that* expensive.

Similarly, whilst glass mirrors would have been more expensive (& more breakable) than their more common polished metal counterparts, I'm dubious about this simple correlation between expense & luck. There were plenty of breakable & expensive items around (including glassware of types other than a mirror), so why only/specifically a mirror?

Both salt & mirrors are considered to have magical properties. I suspect any 'bad luck' associations are more bound up with such ideas as those, rather than a simple matter of cash.

"Bless you" (what's the literal translation of the German "Gesundheit"?) I always understood to be intended to drive out the devils (minor & plural) causing you to sneeze, rather than to repel the just ejected Devil (major & singular).

The trouble with explanations of things like this is that they're often generated by people with more imagination than knowledge, & then they they become self-perpetuating because they sound plausible. I was once sent a page of 'explanations' (the only one I can remember was the alleged origin of the word threshold) & debunked half of them in about 15 minutes, mostly by simply referring to a decent dictionary!