I made Merrill's Best Roast Chicken recipe and my oven was a huge mess

It happens. I'm interested in knowing what others have to say. It's important to wipe down the bottom and sides of your oven beforehand to get up any bits as they will burn/smoke.
I always remove all of the shelves that aren't being used, as those need to be hand cleaned (i.e., are not included in the "self-cleaning" activity); if left in, all that spattered gunk cooks onto them, hard, at the high temperature, adding more work when it comes time to clean them. Otherwise, I use a large, well seasoned baking sheet on a shelf above the chicken for roasted vegetables, in the top position, during the latter portion of the cooking. The bottom of the sheet gets a bit messy, but it does protect the top of the oven somewhat. Wish I could be of more help. ;o)