I tend to suspect that the oh-so-clever accounting techniques and special purpose entities Andrew Fastow cooked up to keep Enron's debts off their books was far more complicated than Ken could understand. (They're certainly too much for my little brain.) But instead of asking tough questions, Ken just shrugged and signed off on them.
Didn't Lay earn a PhD in Economics from Univeristy of Houston? I'd say he was smart enough to fathom the ins and outs of shady accounting practices and off-the-books recordkeeping.