After telling a bunch of different stories about why they fired the U.S. Attorneys, the Bush Administration is not entitled to the benefit of the doubt. Congress and the American people deserve a straight answer. If Karl Rove plans to tell the truth, he has nothing to fear from being under oath like any other witness.
This is correct. The only reason to refuse to be under oath or even have a transcript is because you plan to lie and you want to do it with impunity.
Any executive privilege claims can only theoretically potentially limit the scope of questioning, not prevent them from testifying at all. Working for the president doesn't give you magic immunity from everything.