There won't be an automatic snap grid point in the centre of symbols. The snap grid has it's origin in the origin point, the very point you are attempting to move, and you wanted to move it because it was at the edge and not the center of the symbol to begin width. If the origin is at the edge, there will only be a snap point in the center if it so happens that the distance to the center of the symbol aligns perfectly with the grid size which is just a luck thing. Snap grids are after all calculated as a fixed grid starting in the origin, not magically associated with the image. As Sue says, you'll have to judge by eye.

Effects cannot be trimmed. You'll need to cover it up with something.

Personally, to prevent things from extending through a wall and into the "nothingness" behind, I like to use a mask that is made up from the background with holes punched in it for the dungeon (using a multipoly for that) and put that on top so it basically covers up everything outside the walls, including unnatural shadows.

This is the Delayed Drawn symbols that strikes. This features reapplies symbols after the effect pass to handle certain visual problems, but it only applies to non-building symbols (And with building symbols, I really mean symbols with built-in roof shading, like that tree also has)

Might be better to separate those into different sheets, but you can also fix it by disabling delayed drawn symbols, juts type DELAYDRAWSYM (enter) on the command line, and answer the prompt with 0 for disabled.

Note that this lasts the current CC3+ session, it is not a per-map setting.

You can import them. See this FAQ entry for details:

No, not reinstall CC3+ itself, but the latest update (Currently update 27). The update fixes things not just with the core CC3+ install, but also updates files from the add-ons which is why it should always be installed after installing an update (or at the end if you install several in a row) even if you have installed it previously.

1. When you hit the add-on buttons the program loads some defaults to ensure that things work sensibly. For example, if the overland filters were kept when you went to the DD3 add-on, all catalog buttons would just return empty results because the overland filters doesn't make sense in a dungeon setting. Unfortunately, this also means that the settings loaded with the map is lost. Generally, it is not adviceable to use these buttons in the middle of a map. If you wish to load symbols from a different add-on, use :ICON_CATALOG: instead, this allows you to load any catalog, not just the one provided by the defaults for whatever add-on you switch to. The easiest way to reload the correct settings is to just reload the current map, you don't need to restart the program. Simply save the map (if you have changes you wish to keep), then simply pick the map from the top of the recent file list in the File menu. This will reload the map, and reload the settings from it.
2. One way of making torn edges is to place a white polygon with some fractalized edges on top of everything. The white will cover up part of the map, appearing as a tear. Add some inside glow and perhaps some blur or similar to make it look more like a tear and not just a flat poly.

You can change the default wall to be whatever you want. The :DD2WALL: button just starts the drawing tool named Wall, Default. You can edit this to change the properties (advisable to make a copy of it first to preserve the original, but if you want to edit the tool that is used by the button, you of course have to edit the original, and just keep a copy under a different name as a backup). To make it cutable, all that is required is that it is on the WALLS layer. From your images, you are using SS4, and I don't seem to have a problem with doors cutting that wall though.

To make them into single walls, you need to do some manual work. Erase the entire wall (one of them) where there is a complete overlap, and use tools like :CC2BREAK: to split and cut parts out of walls where you have a partial overlap and don't want to delete the whole length of the wall.

In addition to what Sue is writing above, check for double walls. When I look at the screenshots you post, the door is clearly cutting something, otherwise there wouldn't be a white rectangle there. And the wall fill pattern in the second image is offset from the first one, indicating that there seem to be two walls on top of each other there. (The wall cutting functionality only cuts one entity)

Latest three royals added to the atlas:

It's usefull when you want a set of shades from a different base color than the ones from the default palette.