Arguments against moving to the Plains Hills square
Correct. Can you bet we have copper on that hill?
No, there is no such guarantee. I was simply making a case for the fact that I would rather work that Plains Hills square in the case where the is no there Copper Resource as well as the case where there is a a Copper Resource there, compared to settling on said Plains Hills River square. If it were instead next to a Lake (still getting the Fresh Water bonus) and thus didn't have the River Commerce bonus (from working the Mined Plains Hills square) and if there were clearly nicer squares around it (Tundra Hills and Tundra-converted-Forests are not evocative of this possibilty, neither are Forests or Hills that we cannot see beyond), then and only then would I give settling on the Plains Hills square more weight.
What is it exactly that you are hoping to scout for in terms of Resources by going SE with the Settler? We're all pretty convinced that Forested squares will not contain Resources, outside of rare map types like Arboria that put Deer Resources on Grassland Forest squares (you won't see those here unless they were edited-in by the Map Creator).
The Tundra Hills square is probably just that--a Tundra Hills square. Even if it contains Silver, that Silver likely comes with at least 1 more Tundra square, making it a worse combination than 2 Grassland Cottage squares. I don't see us basing a Civil Service slingshot around a Tundra Silver square--I wouldn't do it around a Desert Gold, either, which is a better square--only on a Plains Gold square or possibly a Grassland Gem square. So, the early benefit of this low possibility would not pay off, in my mind.
What else would we see? Well, we'd see the squares to the S+S and S+S+S of the Settler's initial location. They are both Grassland River squares, that much we already know. We'd get the northern one by settling either in place or 1E anyway, so other than what's on the Tunrda Hills square, it's only the square to the S+S+S that we'd reveal a potential Resource on that would make settling on the Plains Hills square any different.
So, you'd risk losing 2 moves just to see what possible Resources are on a Tundra Hills square and a River Grassland square? That's not a good trade, in my eyes.
Just to show you what I'm talking about, I'll upload a screenshot of what we'd see if only the Settler moved to the Plains Hills square. Although, yes, we'd reveal more squares by way of being able to fog-gaze them, what we WOULD NOT SEE are any additional Resources on those squares. So, we'd only get TWO squares that could contain Resources showing us whether or not Resources exist. Can you really justify the Settler's movement in this manner as a scouting mission? Honestly?