Can I talk you into taking a bag of some sort We usually take a Zimbale saddlebag on metric century rides to add and subtract clothing and carry our own snacks/protein as the rest stops tend to only have fruit and power bars and I can't stomach power bars.
I vote for the knee warmers over tights. Yesterday we had similar conditions. I wore my leg warmers and my wind jacket. I ended up riding the entire way with my wind jacket on but did peel the leg warmers. The DH rode in his knickers, I was wishing I had a pair.