Use :CC2BREAK: to remove the parts of them you don't need. Remember you can use the :CC2MODINT: modifier to get the exact points where they cross.

Breaking a polygon WILL turn it into a line, but just continue working with the line until it is correct, and then you can turn it pack into a poly when done by right clicking :CC2EXPLODE: and picking Path to Poly. If you removed a corner during the process (which you will likely do, you'll need to add in a new one using :CC2INSNODE: to avoid a missing portion of the poly in the corner.

To make the two polys line up exactly against the border of one of them, start by trimming the light poly to the points where it meets the dark one, then make a copy of the dark one, trim it down to just that path segment where they share a border, and use Combine Paths to merge that with the light one, this will result in them both having a border in that space.

Note that it might also be simpler to delete the lighter poly, and assuming it is drawn using a drawing tool, simply use the trace feature of the drawing tool to follow along the edge of the darker poly.

Edit: Beaten by a little mouse, but with a very different approach.

That message shows up because there aren't any wall tools in that style, nothing wrong with your installation.

The forlorn cottage style is a very limited style, and works best when used along with the Dungeons of Schley style, in which it is fully integrated.

If you click the :CC2DRAWTOOLS: button and select SS4 Dungeons of Schley as the style, you can access the full set of tools. The choice is remembered for the rest of the session, so you can use buttons like the wall button after doing that. (Note that this works between these two styles because they are integrated into each other, it won't work with other styles without also first importing the fills from the other style.)

For all practical purposes, yes.

As far as I know, no, it can't be automated. There isn't a macro-version of this command

This button is a relic from the time before styles, where you only had a single "set" of symbols. The catalog buttons are meant to load the catalogs belonging to the current active style, and no styles come with a simple catalog, so it is basically useless (and confusing) on modern installs. Unless you hard-code it to the old simple catalog, it is going to just display an empty catalog list most of the time. If a style does define a simple catalog (The main DD3 style actually does this for legacy reasons, put it is just a pointer to the old catalog, and not really a DD3-version of it) you can easily find it by clicking the :CC2SYMICOND: button yo get all catalogs defined for the current stylke.

You can also open the autosave file straight from the CC3+ file menu (open autosave file) so you don't have to dig around for it. Sorry for not mentioning it in my first reply, but beware that the autosave is only a single file, and may have been overwritten by something else if you have had another map open, depending on your autosave settings (the default is to pop up a dialog every 5 mins so you can chose yourself)

Note that this doesn't have anything to do with the pixel size (or file format), it is the Crop Image to aspect ratio and Restrict image to map border combination that ensures that the exported image only contains your map, and not white area outside it.