"Let say that the FDIC retained 40B of assets in 2008. In principle, the FDIC can only retain assets from WMB and not WMI, right?"
Yes...the FDIC could only legally seize and sell WMB's assets (not Safe Harbor), not WMI's. IMO due to the many instances of co-mingling of WMI and WMB assets, it is possible that some assets in which WMI held interests were seized.
"If the FDIC retains by mistake any WMI assets, will they have to return to the trust?"
Yes they will, but I don't believe the FDIC seized any wholly owned WMI assets but rather WMB and WMB SPE assets in which WMI held a beneficial interest.
"What happens if WMI has interest in some WMB assets that the FDIC retain?"
Once those WMI--SPE agreements were not voided by the Receivership they will I believe have to be returned to the LT, simultaneously or soon after claims higher in priority to Equity are paid in full.
NOTE: The FDIC has approximately $16.2B in claims against the Receivership, $13.8B by Snr and Sub Noteholders and $2.4B owed to WMB. Once those claims are paid and probably some minor administrative expenses, the remains should belong to Equity @ 75%/25%.
(???) As I indicated before, JPM could possibly also be compensated by virtue of being the current owner of WMB, and now successor to any interests they held in those SPE assets (???).
FACTS...NOT EMPTY RHETORIC!!!