SS, that was a very well articulated analysis.
The only thing I might disagree with is when you stated it, "would have been unfair to their Bank's customers and would have worsened their only potential problem: liquidity. Instead of the famous 16B deposit run, it would have been a 20B run. It seems to me they did the right thing for their customers.. keep the cash in their bank."
Correct me if i'm wrong, but the $4B belonged to WMI - not WMB - therefore, it would logically follow that the money did not belong to the bank's customers either. Accordingly, if seizure was imminent and expected - as JPM states in it's answer - then WMI had a right to move its own cash so it wouldn't be seized along with WMB assets. This transfer would represent a deposit liability for WMB and an asset for WMI. I think this is the essence of WMI's claim.