Right, not promising I can help in any way, but at least you/re not being ignored!
Firstly, have you found an auto electrician to take a look at it yet? BCM is probably the worst case scenario, but can't say I've heard of too many - on the cars whose BCMs fail it's usually down to water ingress or other factors.
If not, have you got a multimeter? start by checking the battery voltage - anything less that 12.8V is not good enough - have you checked the connections are secure, and the battery terminals are clean-ish? more often than not most of the electrical gremlins are symptoms of low voltage/bad earthing.
Secondly, check the condition of the earth straps, if they're suspect, undo them and clean them, depending on your car you will most likely have at least 2, one battery to chassis, and 1 chassis to top of gearbox - you can test by rigging up another using a jump lead from the negative terminal to a clean piece of metal on the engine.
Check the fuses on top of the battery, these are often neglected - also, you should find an inertial terminal which cuts all power in the event of an accident - mine is a black box with a yellow button - press that button.
Apologies if this is teaching you to suck eggs btw, as you've been working in that area, I'd suggest its something following your work - you didn't short anything out at all when removing the battery at all?
Anyone in the area with a copy of MultiECUScan could easily check if your BCM is gone.
You also mention other grumbles, it'd be helpful to know what they are, as they may be related, despite not seeming possible - also, please let us know what car and engine you have - in case I've missed it.
Posting from my phone - excuse spelling mistakes