Hi, Windy,
I can't really say that we were consistent in our reservations. We did have reservations for the first three or four days made ahead of time from the US, either by a phone call or email. Even my comically fractured French was enough to get the basic information across -- date and number of beds and name. Once on the trail, we sometimes reserved ahead, as much as several weeks, when we were told there was a holiday coming or when we were hitting a really small place on a weekend. But then, not infrequently, we had to call to change the reservation since we didn't predict our walking pace well enough. I know people who have reserve three or four weeks' worth before starting, and that always amazes me because I just don't know how they can predict what they want to do. I think that's more the exception than the rule, though.
In terms of weather, I've experienced very very very hot all the way to pleasant and mild. Never cold. But I did take a lightweight sleeping bag and was happy to have it. Yes, you're right, the first two weeks are hilly. I was glad to be wearing hiking boots. The scenery is beautiful and so many of the villages are also. One slight "detour" I'd recommend is to take the Valley of the Cele alternative after Figeac. Unbelieveably beautiful and not crowded at all. I think it may have added a day to our total, but it was one of our most favorite segments.
Thankfully, no dog or cattle problems. My only problem after all these kilometers was on the way to Fisterre towards the end of the first day a few km outside the town where the albergue is (is it Negreira?) when a huge German Shephard came bounding out of the open manor gates of a huge estate. At that very second, a car passed, stopped, and took me about 100 yards. A woman arriving at the albergue later that night had been bitten by this dog, and said their owners had kind of an attitude about letting their dog run loose. Her complaint to the police may have stopped them.