Would welcome advice on a town where I could spend 2 weeks in July to make my Spanish flow out of my mouth more fluidly (5th language) and settle into a daily routine of writing. One with street life, maybe on the sea, cafés, all walkable.
Norma
For an answer I suggest a quick scan of the first chapters of James Michener's book Iberia.
Iberia is a good idea for inspiration.
The trouble with the seaside villages is you may hear more British English and German than Spanish.
I have friends who really love Girona, but maybe there is more Catalan spoken there than Spanish. They do not speak Spanish (or Catalan) and manage to get by in English.
I wonder if Toledo might suit you? It is north and at a bit of elevation, so would not be as hot as Andalucia. And they definitely speak Spanish in Toledo. It is a charming, very walkable town, as long as you can manage hills.
Does it have to be Spain? You could do Antigua, Guatemala. It has everything but the beach as well as tons of Spanish classes that are really reasonable. I have a great guest house that I stay at right near the square and it is about $160 per week including all your delicious meals served in the open air dining room that overlooks the volcano. And, you are supposed to only speak Spanish as the owner does not speak English...