I would suspect its not a fuel related issue if you got grey/white smoke. If the fuel stopped or was restricted then you probably would lose power but with no smoke. I'm still leaning toward a belt issue (either snapped of timing slipped), when was it last changed?
Doesn't' look too bad, leaky valve seals would let oil out rather than in. I assume you have tried a new OEM rubber gasket and three top seals? When you fit the cover the three nuts are only just nipped up (10nm IIRC), do it too tight and you may distort the seal.
Have you seen this, just wondering if that could be a potential cause of your oil consumption / loss? That said, I've not trawled through all these pages recently to see what state your pipes are in!
When I was changing my glass over to tinted and resealed the alpine windows I remember there was changeover point where they went from 5 to 4mm (think it was that way around) so sounds like you received the wrong year range glass.
Irrespective of age or capacity if your 11.8v was an accurate battery voltage reading that indicates that the battery was flat at that time. Unless you had the lights on for a long time while adjusting them then its worth keeping an eye on it as you may have something draining it.
As above 11.8v is dead, my 300 has a main fuse box in the engine compartment with the big stuff in it (60A and 30A fuse links) which are the main supplies, however I suspect a flat battery is the main issue
Time to find another garage, they are just fobbing you off. If they can't change brake fluid then they aren't a garage as that is bread & butter stuff. Disks won't be painted, however they may have some form of coating to help the pads bed in. Bedding in the pads won't affect their ability to...
Have you checked the main fuses in the engine compartment, sounds like they would be suspect? Did you disconnect the battery when fitting the lights? may have left a connector loose?
Also worth taking the timing cover off to check the timing (including the cam timing) properly, may even be worth throwing a belt kit in there once you know if it is a good engine or not.
Have you checked the oil pressure (with a gauge - don't just rely on the light) as low pressure will kill turbo bearings along with the mains / big ends