Ok, here we go.
The Adeptus Mechanicus controls forgeworlds all across the galaxy. These act as manufacturing facilities for the imperium. All forgeworlds maintain a PDF, same as any imperial world. The main PDF forces are made up of Skitarii regiments (these used to be called tech-guard regiments), made up of mechanically augmented humans and organized like guard regiments, but with a high number of tanks and armoured vehicles. Most forgeworlds also maintain a Titan Legion. Originally these arrived with colony ships from Mars during the age of strife; many more have been formed since.
Therefore the Titan Battlegroups fall under the control of the Adeptus Mechanicus. Imperial commanders can request the assistance of the Titan legions through the Adeptus Mechanicus (I'll call it AM from here onward)and, if granted, a battlegroup will fall under the command of the theatre commander for the duration of its involvement, however it is almost always the case that the titan battlegroup commander becomes a part of the commanders staff, answering only to him and the AM. If the AM withdraws its support then the titans will leave, the AM control supersedes the theatre commanders.
As to the FW models, it'd have to be a really good sculptor to match the detail on those! I've seen one up close and still not lost the enthusiasm for them. Yes its a big-game only thing, but so are superheavies, you need your opponents permission to use them too.