You certainly touched on a lot of sujects although I am not certain for which answers you are looking. Let me try to answer one point very quickly.
I have no knowledge regarding D: Factory Image. I only know its supposed provide the original Windows 7 files and folders. The files I saw in there when I made the system operating files visible, had no idea regarding any of them. Somebody else will have to tell me what is in there. I just did what I suppose to do and got out.
The factory image on D is exactly what the name says - it is an image of the OS at the level that it was installed at the factory. You can use it to reinstall your system at exactly that level.
If the factory installation of the OS was a while ago (e.g. your PC is already a few years old), a reinstallation will trigger an update of the OS with all updates and possible SPs since that time. That is a real pain and can take quite some time to process.
Therefore we recommend frequent imaging. Then you can pick a much later state of the OS thus avoiding all those updates (that were already done) plus you save all the program installations you have done since plus all the data you might have on the OS partition.
The only point I would make is: DO NOT USE WINDOWS IMAGING !! Too many problems can happen with that. Use Macrium which I find to be the easiest, fastest and most reliable option.