I’m not sure what the straw is believed to be accomplishing in cob construction but I can speculate. First, in the dry clay, the straw is protected from many of the things that would otherwise break it down, so it probably does retain some structural utility. Since straw is hollow, it can also provide some insulative qualities. Even if it did ultimately break down, by then the clay would be dry, so the air spaces would remain. Finally, I’ve been told (haven’t tested it) that wet straw can have some adhesive qualities (like oatmeal does). If that’s the case, it would definitely help to hold an unfired structure together. (BTW, I’ve heard of
people building the whole structure of clay, then setting a bonfire inside to dry and harden it. You’ve probably come across that too, but I just thought it was so cool!)
Cob is really messy looking to me, with all the whiskers sticking out everywhere. Chopped straw might mitigate that somewhat, but I’d be careful not to overdo it with the straw. Too much could weaken the clay. You can also add sand (not too much... I’d start around 15% by volume). That can modulate expansion/contraction as well. And for the whiskers, I’m sure a propane torch could help with that.
I missed the bit about the limestone screenings. Clay is really special stuff for something so common. Its individual particles are plate-shaped. This gives it its plastic qualities. It can only be formed by long millenia of erosion and shaping by the waters that carry it to its banks (where we ultimately find and dig it up). That’s why the porcelain and kaolin clays are less plastic. They’re found close to the places where they were first eroded, so no long trip down the river for them. Bottom line though, nothing but clay will work for cob. Nothing else has those properties of malleability and the power to hold the shape you form it into.