I like your logic - it makes sense.
However! In threads on the forum when things don't happen at all on the back door (window/latch/wiper), I feel its generally a component that is faulty and stopping other components from working. Everything goes back via the CCU, so, for example, pull the switch, the CCU drops the window then pings the solenoid. If the window won't wind, it won't ping the solenoid. In fact, if the switch isn't faulty - its almost always the window regulator.
When things behave strangely, I feel its often the wiring.
You're right about relays in the boot. There are 2 (or a double unit) there for lowering/raising the window. I don't know about solenoid power.
You could very well be homing in on the right area.
There are/is earths in that boot area. IIRC there is a common earth in the lamp unit for the bulbs that is prone to corrosion and causing strange behaviour.
The Rave manual is an awesome tool for diagnosing these types of faults. There are full circuit diagrams that are pretty easy to follow. Different sections of wiring have their own diagram, the wire colours are documented as are which pins in which connector they go to. The best bit though is that there is a picture of every connector - so you can see exactly where it is (once you've put the pic into perspective). These connector pics also include earthing points.
I'm putting this ere so I can link to it. If you can see adverts on this page selling car repair manuals, DONT click on them. They're sneaky adverts from google which detect words used in this fred, on the chance you will be attracted to click on said advert. The adverts are trying to get you...
www.landyzone.co.uk