For a relativist, your arguments are very absolute.
> The problem just is that just like no two persons have the same idiolect, no two persons have the same perception of the "language as such".
No, that's not "the problem" because that's not a requirement for anything I said.
> We learn it by example
Initially, but not solely.
> You are entitled to your view of "since", but what if I ask you how you know it's wrong to use it for evidential causality marking?
That answer would sum up in a combination of definitions, etymological influences, and a reasonable purpose for the term. It's also a longer answer to explain than it is worth right now, given that I'm positive you would already know the answer if you bothered to think about it honestly for a few moments, regardless of what postmodern-flavored hand-wavy disputations you might raise to the contrary.
> What if someone says that "due to" can't be used except for describing money owed (an old teacher hobby horse), and that it can't be used to mean "caused by".
What is "due" or "owed" is not solely about money, and you know it. Money is context -- not definition. The monetary meaning, in fact, proceeds from the owing definition -- not the other way around. Further examples of yours obviously intentionally We get to a slippery slope when we start saying which change already happened must be rolled back.
Maybe so -- except that you have not established that the change under discussion has already happened. It "is happening", arguably -- though it's difficult to establish even that, when the possibility that there is just some eddy in the stream that results in what you seem to think is a sea change. In fact, my experience is that some of the assertions you are making about how people actually tend to use "since" are just wrong in some of the particulars you present.
