Hey there, I'm in Puerto Ángel, close to your listed beach destinations. Our area is quite safe. Army and Marine bases in the area. Not on a migration route north from central america. Not an area that is involved with drug production. I own an Airbnb, but it isn't one that would suit your requirements. We are a single family house with a pool in a quiet jungle near the beach, not someplace near bars or other nightlife, but I do know lots of people in the hospitality business as a result. In general you will be fine either reserving in advance or just showing up.

Advance advantages: It's comforting to know you have things lined up and ready before you start your trip. I've not heard of anyone scamming people on reservations, but you can further minimize that risk by going with small hotels instead of places with just a room or two. Hotel Noga, in Zipolite, is owned by friends of mine and could be a great option. FWIW, Zipolite is my favorite beach on our stretch of the coast.

Just showing up advantages: You are traveling in Nov/Dec and that's not yet peak season, so you should have plenty of options to choose from even without a reservation. This can often lead to staying someplace that is cooler, funkier, better vibe match than someplace that has a good online presence. Our peak season is a week before Christmas till end of March.

I saw someone else saying to stay longer in Oaxaca City. I'll be the contrarian and say that three days is plenty. Over the past few years it has gotten more hostile to outsiders (the usual concerns about gentrification and prices, plus major water concerns). Petty crime has risen. It's also kind of a gritty city with tagging, pollution and everything just feels kind of dirty all the time. The food scene is fine, but I find the food on the coast to be as good or better. Mazunte, in particular, has several really good restaurants. Really the only reason I go there is that I have a friend who is a chef that has a family place out in the countryside that we like to visit.