I ended up guessing after no one found anything, and later used an online solver (https://mrgris.com/projects/minesweepr/) to prove that there was in fact no safe square. BTW this solver is pretty cool (and different from the one I linked in the other thread), it's more of a thing to play with and learn from than a tool. The links to use it (live demo / board solver) are on the right side of the website. Check it out, even if just for a fiddle!

Edit: I did have to input the whole thing manually, by which I mean just a cropped part that includes the unknown territory.