Fannie Mae’s decision to begin punishing people who walk away from their unpaid mortgages could prove difficult to sell to the public and might be impossible to execute, housing and lending experts said Thursday. But it was unclear, the experts said, why Fannie Mae was threatening delinquent owners and what it hoped to achieve…And there were basic questions about how Fannie would be able to distinguish between those homeowners who defaulted intentionally and the unfortunate ones who had no choice.