I hate to throw cold water on the situation...but with everything you've tried and still get no start...and if I understand correctly...when you turn the ignition to the start position...you hear & get absolutely nothing (no clicking from the starter, or "urr, urr, urr" from the engine's attempt to turn over)...I think the issue may be with the EWS system.

I've seen this happen before on either an e38 or e39 forum...where the key was left in the ignition (just like your situation)...and the battery was drained. The issue turned out to be that EWS got out of sync. With the key being in the ignition and turned to position 1 where some control modules are initiated...then the battery draining and losing power...yet the key is still in the ignition where communication with the transponder could have been initiated as the car's battery voltage begin to drop...EWS could have gotten out of sync.

It takes BMW DIS to resync EWS...it is not like the FZV/DWA features of the key that can be reinitialized in your driveway.

BTW...the dealership gave you some incorrect info...the battery inside the key has no affect on it's ability to START the car. The battery is only required for FZV & DWA (remote locking/unlocking of the central locking system & arming/disarming of the anti-theft alarm siren system).

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