I can't answer the question about reservations, although I never heard of them being used in Spain, but I can tell you that restaurantes have no problem with the appearance of a single diner. A few years ago, I was in Madrid for a month by myself (the last few trips my husband accompanied me). I went to eat...obviously, by myself. How did the restaurantes treat me? The same, if not better, than when I was accompanied. They seemed to want to take care of me. I didn't go to the same restaurante everyday...but did once a week. So, the waiters got to know me and treated quite well.
Sheryl