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.

You're really into making this garden leafless?

The full color logo is actually quite nice, it gives the impression that this piece was important to get right, and warranted more attention.

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.

Simple wooden bridges (whole, partially ruined, and ruined)

Tents/market stalls

More options for paving