You really need to read the User's guide first. It's not that big.
To understand walls, floors, and levels you need to read the manual "Levels Explained" (https://dodecagon.nl/dodecagondocuments). Your outside wall of 950cm is not a problem but keeping it to each level avoids some other problems that might occur. The texturing looks the same either way.

But apparently this means you can't really add a 'wall' to represent siding after you're finished measuring and painstakingly entering all your measurements.

A wall is ... a wall, not siding. You can set a baseboard on a wall with a given thickness to represent siding or you color/texture the wall itself. If you want to physically add a separate siding you add either a second wall or other objects to function as siding but this will unnecessarily complicate your design. Since you need the holes the best way is to use the existing wall and texture the outside as siding. If your siding has a thickness you add that to the thickness of the walls. Remember that Sweet Home 3D is not a CAD program, it's for house and interior design, the max precision is 1mm.

For changing the colors or textures of multiple objects there are two plugins you might find useful: General Change Texture Plugin and Change color. The easiest way to install them is to first install the Plugin Manager plugin . Direct download (v1.5.3): PluginManager

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