Philip, I don't think any of the steep sections you mention are anything to be especially worried about, so long as you just take some care. The descent into Zubiri was quite slippery the day I did it, as there had been quite a lot of rain that day. (We left Roncesvalles in the morning in torrential rain: people with little $2 plastic ponchos were soon to find they were not suitable for the conditions!!!) However, I used my stick for balance down the hill into Zubiri and was fine. It was at the bottom itself when I reached the concrete path and relaxed that I had what was nearly my trip-ending fall!!!! I had been walking for weeks from France by this stage, without an incident. But I slipped, completely lost my footing and fell full frontal, spread-eagled onto the ground. Amazingly, apart from a few twinges that evening, I was fine the next day. (I think Our Lady of Le Puy was doing her darndest to look after me!!!)
As for the busy highway crossing near Leon, I hated it, and found it very dangerous. A few months ago Rebekah reported that it seemed like something was going to be done about it, but I didn't think it could have been fixed yet.
Margaret