Ah, I see where I screwed up, I mixed the final turn and final angle in my head, and it came out a bit jumbled. I'll fix it, sorry about that.
Yeah, its a good puzzle. Flat Earthers aren't as dumb as the media makes us out to be.
Depends on who you ask. Also:
I see no reason your definition invalidates anything. He walked ten miles due south, turned ninety degrees east at 89.85N longitude. He walked for ten miles due east. He then turned ninety degrees northward and walked ten miles, completing an angle that isn't ninety degrees. To clarify, if he started walking south at zero degrees latitude, he'd have turned northward again at sixty degrees east latitude.
Ah, I see where I screwed up, I mixed the final turn and final angle in my head, and it came out a bit jumbled. I'll fix it, sorry about that.
My point applies. He still wouldn't have ended at ninety degrees east latitude, only sixty degrees. And you'll notice that, yes, the angles of the triangle are in fact more than one hundred eighty degrees, so although not a triple right triangle, it does demonstrate the point. I'm with you that such a huge distance need not be covered, but a fairly large one does for it to be relevant.
You're right in that it's possible at a smaller scale, although not with the exact same results, but a scale of miles is still enough for curvature to be present easily. If you like, see the infamous ENaG for it's chart of drop distances on a round Earth.