This is Plan A. I'm flexible. Just want a good swimming beach, and a chance to play music with locals and other travellers. Start in Mazunte, on the coast. Explore small beach towns. Find a good swimming beach with a friendly community to stay for a couple of weeks, bus up to Oaxaca City. Stay for maybe a week. Go back to the beach for a while, fly out of Hualtuco or Puerto Escondido. 

I've spent a lot of time in Mexico. My Spanish is not fluent, but it's functional. No problem if I need to get by in Spanish only.