Well, it's not like he had to teach the Basilisk a full Hogwarts curriculum; he only had to teach it what he knew that triggered the Interdict of Merlin, which is only the top whateverth percentile of his repertoire.

Fair point. I'd forgotten about the Interdict. With that said (and this applies to your second point as well), it seems unlikely that the Interdict of Merlin is the only reason for knowledge to be lost over time. For example, Riddle apparently found the horcrux ritual in a book, and that seems like powerful mostly-lost knowledge. Also, wizard society generally seems much worse at knowledge maximisation than muggle society. (side thought: is there even a single mention in either canon of wizard universities?)

The Basilisk may not have a perfect memory as an animal, but it "would be huge flaw in sscheme" if Salazar's magical Parseltongue knowledge was corruptible by the limitations of any old snake's brain.

True. One has to wonder, generally speaking, just how the whole thing worked, given that Parseltongue seems to blur terms for which it does not have an exact parallel ("schoolmaster", "hourglass to move through time"), and that seems like it would be a problem for advanced spell instructions.

I think you're extending your computer analogy too far. Salazar didn't have a revocable password to the wards, he knew the magic that created them, and the rest of the Founders certainly did not have the power to revoke spells from the Source of Magic.

We don't know that. The four founders came together to raise Hogwarts in the first place, suggesting that each of them knew only some of the magic necessary. There is no reason to believe that Salazar was the one who knew the magic for the Hogwarts wards, rather than, say, Rowena.

Additionally, you don't need to revoke a spell from the Source of Magic to prevent someone else making use of it. Going back to the computer analogy, being a system's original programmer doesn't mean you can automatically hack into any instance of that system. It is worth remembering that once Salazar left, it would have been three magical prodigies against one in the matter of establishing Hogwarts security.

Likewise don't get me wrong, I think it's reasonable to assume that whoever got the basilisk's knowledge got at least some very powerful magic from Slytherin; I just don't think we should overestimate how much that was.