You form long marching lines in those times (pre-Medieval) at your own peril. Consider this:
https://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest
While this isn't in a mountain range, abandoning any kind of battle formation could always lead to sneak attacks, even by inferior foes. You might be able to give a command like "strategic movement", but then you would end up with a huge defense
malus as opposed to a bonus in the mountains, so perhaps it is battle formation in the mountains that you'd need Mountaineering for (making the other side of a mountain range an excellent defensive terrain before that point).
So you feel the game would be better if we made it possible to always traverse mountain plots and change the defense from a positive bonus to a negative penalty?
I hugely disagree with you there, it would take away a lot of the strategic element that I like about mountains in the game.
I rather prefer to think that a mountain plot is actually, in its entirety, a mountainous area and that the mountain foot (the closest place where it's not crazy to send an army without specialized mountaineering equipments and steady supply lines) is found somewhere inside the neighboring plots (unless they too are mountain plots).
If a mine is positioned at the mountain foot, by that it's so close that people would say that the it is mining the mountain, then the mine would in-game be positioned on the hill or flatland adjacent to the peak plot. If the mine is, I don't know, about 3000+ meters above water, and is working harsh environments with no local population to lean on, then it's positioned on the mountain plot in-game.
If a bonus is positioned on a mountain plot in-game it is located at a high altitude and harsh environment by default, to serve the game mechanics we want in the game. If it were located in a low altitude non harsh environment then it would have to be represented in-game on the adjacent flatland/hill.
Sorry, but other than the fact that pretty much any rocky planet will be scarred by asteroids sooner or later I cannot see support for such a strong claim.
I wouldn't say that it's such a strong claim to say that a planet need terrain to support earth-like life. If a rocky planet is perfectly smooth then it can't have moving tectonic plates, and then it probably doesn't have a hot liquid core and volcanic activity either, such a planet would probably not have what it takes to support earth-like life.
That doesn't change too much. You can still end up with one small tile surrounded by eight big tiles. Non-square tiles can reduce this problem a bit, but in the end this is still a problem (especially the fact that these eight big tiles ought to be neighbours toeach other in that case, which isn't modeled in the game).
Maps in these games seem to be inspired by the Mercator projection, which doesn't care for surface area but for angles (see
https://en.wikipedia.org/wiki/Map_projection). One degree in any direction is shown as the same distance, no matter the height profile.
I could also add that I don't think of the plots as having strict borders with it's neighboring plots, they bleed into each other.