Since I am not aware of any major discussions about this topic (some veteran might chime in and provide some directions, you never know), I can only provide my personal opinions.
1) I fear it's not the answer you want to hear, but you always make a map for a purpose. If you plan to have your map printed in a book, you better make some design choices to reflect that, opposed to pure digital publishing.
In practice, it means you choose a zoom factor it is best viewed at.
Printing will use different "zooms" than website publishing, especially when it comes down to legibility of the labels.
2) You have many options. You can use colors, you can use all-caps, you can use italics. You can even use other fonts, but I try to avoid that since it adds a level of chaos to the map that is not necessary. Just give it a try and see what you like.
3) I use one rule here. (Can't say I didn't break it in the past, but at least I try to avoid breaking it). Don't make a label bigger just because a place is bigger.
Instead, try to find a good average that serves your map and try to keep to it. People will look for consistency. If all your mountains are labeled with the same font, in the same color, the same size, and the same font variation, people will recognize mountain labels more easily.
4) I try to keep the same terrain types at the same luminosity throughout my maps. This makes labelling a lot easier. If your style differs, try an outline around the text, which might keep it readable.
Just some suggestions,
I hope this helps.