Good questions; I'm just finishing up clinicals; so I'll answer them the best way I can.
As far as grading clinical coursework goes, there is an evaluation form that you provide to your preceptors with which they can provide you a number grade in certain categories (very arbitrary). Sometimes, the rotation itself has a written exam (either midterm and final, or just a final) attached to it; and the preceptor will take that exam grade into account while figuring out your final grade on your evaluation form.
However, MUA is pilot-testing a new clinical shelf exam scheme whereby at the end of each of your core rotations, you would take a 2-hr shelf exam (USMLE-style) at a prometric or other similar testing center. I'm not sure how this will affect your score; but it's a good idea IMO. Not sure when it's going to be required, but it isn't right now.
As far as electives go, you are graded by your preceptor on how well you perform on the rotation itself; no exam usually.
I don't think there is any attrition in clinicals; you'd have to be pretty incompetent to fail a clinical rotation. Whether it's easy or not depends on where you go, and how much you want to get out of it. Some people just cruise through rotations doing the minimum amount of work needed, and some work harder to learn more. If you really want to learn, I'd spend atleast the first 2 weeks of the rotation hitting the books for that topic, and then applying it in the hospital.
Let me know if you have any other q's.