Bingo!
Downloading a separate factorio.exe (via a .zip file, in my case) does allow you to work around the problem, of been unable to launch Factorio, through steam. But this doesn't solve or identify the issue itself, its only a work around.
Looking over daniel34's post, and reading the associated ...

Ok, I deleted the contents of the Appdata\Roaming\Factorio folder (including the save file), uninstalled the game, reinstalled it, under the last stable version (0.14.x), and everything ran just fine. I then re-enabled the beta version (0.15.9), ran the game and the same problem occurred, the game ...

Ok.
After after acquiring the 0.15.9 zip file, from a "lengthy" download process, I exported the files to a folder in my documents, launched the game via factorio.exe and "The game appeared!" Fully operation! :D


Yes, it would appear running Factorio 0.15.x by itself works just fine, where running ...

:lol:
Upon reading the response from Koub (Thanks by the way), I have now realised numerous errors in my original post.
Yes, my problems are specific to 0.15.x, not 0.5.x. (That's just hilarious)

Anyway, from the Appdata folder I have the current and previous logs. Upon launching Factorio now in ...

The Problem
This problem started for me, once the [0.5.x] branch was released (and applies to it only)
Upon clicking play in the steam library, to launch Factorio, the "Preparing to launch Factorio..." windows appears briefly, as you would expect.
The beta version number "[0.5.x]" next to ...