"MIFLAW - "I was sat" means that something external put me into the seated position, if it was me doing the putting then a simple "I sat" would suffice." No, I dispute that. I refer you again to contemporary French grammar - my contention is that this structure is a hangover of Norman French and Anglo-French. "Je suis assis" means, literally, "I am sat" but this is the nearest equivalent to "I am sitting". So it would not surprise me if, until the rule making frenzy of the 17th century, both were accepted forms. For someone then to say, on a whim, that it is NOT correct doesn't cut it, I'm afraid; if it is maintained in common usage, then it is correct. (Of course, if I have imagined this French heritage, then I am more than willing to be corrected.)
It cannot IN ANY WAY be compared to walking or driving because these verbs describe activity whereas "sit" describes a stasis. The best comparatives would be "stand" or "lie". Interestingly, though no one says "I was lain" (the only possibility is, "I was laid" which DOES mean someone else is involved), lots of people do say, "I was stood there" to mean the same as "I was standing".
so perhaps, in this case, rather than work out rules that don't seem to apply universally or analogies that don't work, we should settle for observing widespread usage? It's not as if (unlike "literally") we're changing the only previously existing meaning of the word; or (as in the case of "between you and I" or "off of") we are using something that can be "proved" wrong by analogy with other grammar rules (as I say, I do not find your "walking" analogy convincing or sufficient.) All we are doing is acknowledging that there is more than one way of saying the same thing, both with a substantial history and pedigree, that doesn't confuse the listener or remove a useful distinction.