Ugh, that sounds just like my 9-month-old! Diaper changes are a nightmare with her. I stay home and therefore do most of the diapering. My DH had to change her the other night when I was out and was saying it was nearly impossible! My strategy has been that we have a little drawer in the changing table, and anytime something baby-safe is no longer useful (like sunglasses that lost a bow, a small flashlight that doesn't work) I put it in the drawer. Then during diaper changes I can whip out a distraction. And the best part is that I swear even the littlest ones can tell what's actually a toy. So she knows she's getting something special that's not made for babies and she relishes it!
She's also been whipping her head back against the high chair, usually when she's done with supper. I've been thinking it's a couple of things with her. One, I need to step up the signing (a few basics like "more" and "all done" can be lifesavers!); two, she's not napping very well during the day, so she's very tired at that point and she tends to get more squirmy and even bonk her head in frustration as she gets tired. And three, she sees her brothers up and moving around and wants to join them, now that she's mobile. So as soon as I see that behavior start, I try to get her out of the high chair so she doesn't hurt herself and also so she will be able to communicate her needs and understood that when she does, they will be met.
Good luck. It doesn't last forever, but it may feel that way!