I think both "fell" and "dale" are inherited from the Vikings.
I'm not an expert on Scotland, but there is a general right of responsible access, rather than the system of "rights of way" in England and Wales. I theory that means you can walk anywhere, but in practice you have to research where there are physical barriers like fences so it's harder to plan routes on a map.
I'm not sure what the wild camping rules are there. There is a lot of wilderness at altitude, but the walking can be rough, wet, and plagued by the dreaded midges in the west in summer.
Dark green - probably forest but it depends on which type of map you're looking at.
The Munro is any peak over 3,000ft, regardless of terrain, but the tree line is lower thank that everywhere, I think.
There are trails - long-distance routes - but may places without them.
Scottish laws on naturism are broadly similar to Engish but framed differently, I think. You would normally only get into trouble in populated areas.
I'm happy to be corrected on any of that but those who know more!