Hi, Pieces,
You will love the Primitivo. I may be wrong in my assumption that this will be your first Camino, so if some of this is elementary, sorry.
15 days is, IMO, a lot of time to walk the Primitivo. I walked it in 11 days as a 58 year old woman --I'm in decent cardio shape, but still. Here are my stages:
camino-primitivo/topic4841.html
If it's roughly 300 km, 15 days would come out to approximately 20 km a day. That's a low daily average (of course, I haven't figured out whether the spacing of albergues makes that possible). There is only one day with any significant elevation gain, and that's the day you choose Hospitales or to go up from Pola de Allande. It's about 600 m total ascent, and if you've done much hiking you probably have a sense of how your fitness level jives with that number.
My very impressionistic calculation is that the Primitivo is one of the caminos with the least amount of asphalt, there are lots of wonderful wooded and off-road trails with the occasional country highway thrown in.
There's a lot of rest day discussion on this forum, which you can easily find with a search. Basically, it boils down to whether you feel out of sorts or rejuvenated when you stop and "do nothing" for a day. I fall into the former camp, though I frequently do take rest days if my companions want to. One compromise is to do a very short day either into or out of your rest day destination -- that way you have plenty of time to see the sights and still get some relief from the daily pounding.
I wear North Face Vindicators, which I dearly love. They have all the tread needed for any camino I've ever walked on. They are not high boots, a little higher than regular running shoes, and they are the perfect fit for me. I'm not sure which North Face model you're describing, but if it has a sole like the Vindicator, you don't need to worry about having an extra pair. I do bring sandals, not for walking but for shower and roaming the towns at night, so I wouldn't give up on that idea either.
Buen camino, Laurie