Very nice. Nothing can beat a touch of certain death :)

Different kind of tiles basically. Right now, there are only cobblestone (clean and dirty, but as Quenten mentioned, overgrown versions, things like tiles, loosely-fitted flagstone paving, etc.

1. As @Lillhans pointed out, your edge isn't straight. I don't know if you started with the left or right before mirroring, but none of them are straight, so it isn't a mirroring problem, it is a problem with the original shape. You can't mirror a non-straight edge and expect it to match up (unless the mirror line itself isn't straight). There is a 0.03 map unit horizontal difference between the top node and bottom node.
2. At any point really. Either use the Make Varicolor command on the entities before defining the symbol, or bring up the symbol for editing after defining it and us it on the entity. I do note you have attempted to define a symbol as varicolor in your map, the COSFLAME-VOL symbol, but here you have made the mistake of placing a symbol definition inside another symbol definition. Never do that. A symbol should be defined from base entities, NOT other symbols. Either use Explode on symbols to break them down before defining new symbols from them, or if you just need a copy of a symbol, for example to make vari, use the clone feature in the symbol manager.
3. Save your map with the new symbol in it. Then open up the symbol catalog you want to insert it into in the editor, and go to symbol manager, hit Import, and pick the map with your symbol in it.

It means you have selected the entity to trace at the wrong point. The trace will always go through the point on the entity where you selected it.

(Note that it might go the wrong way around untill you have moved the mouse enough from your starting point to pass the selection point)

Yea. The default behavior of drawing tools and symbol sis to pick their own sheet. This can seem weird/annoying at times, but generally one tends to end up putting things in the wrong places much more often if they didn't do that, so the net result is that this behavior is quite nice.

Note that (almost) all these tools are prefixed based, for example the landmass tool in most overland styles look for a sheet name starting with LAND. If your current sheet does so, it stays on that, if not, it goes to the main LAND sheet. You can use this when you create new sheets to get the tools to put things where you need it.

Are you using "change like draw tool" when converting those entities? If you just use change properties and change the fill, you won't get that automatically, but if you use the change like draw tool command instead (found by right clicking change properties) it sets up things correctly.

As an alternative, you can also set up an outer glow using effects, that can often give a neater result (but not the exact same though)

And after gaming with you for a while, they became even more careful? :)

Wyverns maps are now to be found in the atlas:

Looking nice. I love the look of your cave, and also where it breaks into the dungeon.

This set of maps have now been added to the atlas. Thanks for the great contribution.