Some symbols are fitted with a shadow as part of the symbol, so be aware that this can make them look weird in comparison with symbols that haven't been mirrored though.

Without seeing any images, this is an appallingly difficult thing to try to provide advice upon, because it's impossible to guess why you're not happy with what you've managed so far.

Subterranean silver mines in ancient to medieval settings (presumably what you'd be aiming towards for a fantasy setting, and assuming you're not opting for some form of surface/open cast mining, which would be an alternative) essentially looked a lot like mostly narrow, often straight, low caves. The nature of these will be heavily dependent on what state the silver is in. If it's in the rarer metallic veins, the narrow, tunnel-like caves would be appropriate. If it's in the commoner form found on Earth, an ore combined with lead or copper, which needs further processing to extract the silver, larger areas of higher-grade ore might be opened up as a cavern. In addition, processing facilities would almost certainly be constructed on the surface nearby for the latter ore-type mines particularly (or perhaps underground where undead workers are being used, as here), which would make for a larger, more varied complex overall.

The mine needn't require vertical access, as many early silver deposits were extracted simply by starting to dig a tunnel into a hillside more or less horizontally. That would be an option for a more 3D map, however.

I don't know. What you've done has the advantage of real clarity, and to an extent, it reinforces the "area of the unknown" the events took place in, given the river valleys were the only places relatively rapid travel was possible.

For the problematic mist, you could try drawing a few small polygons in a suitable colour (grey or white, perhaps) on a Sheet that lies above those already used for showing the background and symbol(s?) for The Realm of NeverBe (so nearer the MAP BORDER Sheet than the COMMON one in the Sheets list), and then apply some Effects to that Sheet, such as Blur and/or Transparency, to get the polygons to fade and blend more like mist. Using different shaped polygons may help with this too.

Unfortunately those electronic watermarks can be removed by someone sufficiently knowledgeable. I've chanced upon pirated copies of a few RPG products from the OneBookShelf sites (Wargame Vault, et al.) where they've been taken off; obvious, as the digital products aren't available anywhere else.