You identify JPM as a potential receivers of Fannie. Which do you suspect has the riskier portfolio: JPM or Fannie? Based on bank secrecy laws we will never know, of course. You could say JPM is more diversified, perhaps, but Fannie's operations are relatively plain vanilla relatively speaking. Also, JPM has the HUGE special accounting privilege (accorded only to banks) to mark its assets to market, selectively. In other words, even when JPM reflects viability, you can never be sure. Give Fannie, or any other entity the ability to selectively mark to market and no problems will ever exist---on paper. The real world will be a different story entirely!
There you have it. Fannie is exempt from SEC laws concerning transparency so it is again impossible to know.
Of course, once risk reaches a certain level it ceases to be a liabilty and becomes a negotiating tool.