Hi Richard:
Those look good, and are likely to work well in  FS ! 
 
Regarding the Rectangle tool, what I meant was IMO, it's preferable to make buildings with rectangles initially drawn on the ground, extruded to blocks, and any interior walls  then being made with the Offset tool... versus making the walls with the Rectangle tool.
That method can help minimize the challenges with texturing interior walls that are not parallel to the outside wall, which can create multi-planar Z-buffer artifacts and/or problems having them still visible after imported into MCX.
However, the Rectangle tool, IMHO, becomes the 
tool of choice when making "textured" 2D doors and windows on the exterior walls of buildings, as that is a preferred method over actual 3D framed and/or transparent doors and windows which could have a cumulative impact on FPS.
Extra 3D geometry for 3D doors and windows, transparent window panes etc. are as Phil Taylor of ACES once said, "computationally expensive" to the FS rendering engine, and IMHO (like some library and autogen objects) should only be used as a visual asset when it will be seen close up and/or while taxiing around on the ground nearby.
If we look at most 3D objects in FS, they usually have the windows and doors "baked" into the texture, and those textures are usually wrapped onto the building in pieces that fit 1 wall at a time (or are even able to wrap the entire building from "1" big texture image if pre-configured in one's graphics utility).
Some folks may construct the building in full 3D detail as you had, then use a photo-realistic illumination rendering utility (or simply use a graphics program) to put shadows  where they would normally be seen, taking screenies to create the texture sheets for later use as materials.
Those detailed original 3D models can then be copied and their geometry 
reduced  to minimize vertex count; when the textures are applied, the object may look adequate for the desired effect in an FS scenery relative to where one's aircraft viewpoint might see them.
I agree that Sketchup's Materials GUI is confusing at first; that was one of the 
few things that really frustrated me about Sketchup when I first began using it. 
 
If one imports and scales a texture for use, as soon as even "1" parameter is changed, Sketchup applies that change to all 
existing and subsequent mapped uses of that same named and defined "Material".
So one must create a new copy of that Material after it has been mapped anywhere on one's model 
if even "1" of its parameters (other than positioning) is to be changed. 
 
Before you know it, you've got multiple copies of a particular texture in your Materials collection.
 
<
Sketchup Materials are like coat hangers... they multiply when you're not looking !> 
 
Thankfully MCX deals with this multiple texture file / Material issue when it imports one's model by compositing them as images onto a minimum of FS texture sheets to reduce draw calls and file I/O. 
 
One of the advantages of creating one's buildings with wrap around textures is one can make a 
night texture ("aka "light map" with a *_lm.DDS 
or *_lm.BMP file name structure).
One can edit the desired lighting in the doors and windows on the exterior walls of buildings using a graphics program and put that alternate texture in the \Texture sub-folder along with the day texture; then FS can 
automatically swap the day texture file with the night texture (having the "
_lm" file name suffix)  between dusk and dawn. 
 
I think you are on the right track... keep up the good work ! 
 
GaryGB