I used my crocs once when I had some bad blisters. I was back packing, and they were the only extra shoes I had. I took the insoles out of my boots and put them into the crocs and an extra pair of socks. It improved the fit, but they were still loose. I have pretty strong ankles, but because of the fit I had to be careful not to roll an ankle. They do not have good traction, and can be quite slippery. The soles are not spider rubber. I think Burgos is the start of the meseta. Looking at the elevation charts it should be a little flatter.
What treatment did they suggest for your feet besides rest? They sound raw and painful, but a soak in epsom salt (sulfato de magnesio or sal de Epsom?) may help to dry them out and heal. A little provodone iodine, or betadyne in the water may help reduce the possibility of infection. The soak may sting at first, but should get easier. Make sure the bandaging and tape are NON STICK. Make sure they stay clean, dry, and get as much air as possible. Are you planning on wearing socks with your sandals or just bare feet? Socks may offer a level of cushioning, keep some of the trail dirt etc. off of your feet, and provide some protection including sun protection. I would change them a couple of times during the day with a clean dry pair. You might be able to give the dirty ones a quick wash at a rest stop and hang them from your pack to dry after washing, so that you always have a clean dry pair in reserve.
Make the most of your rest days, and keep us updated with your progress.
