The only thing I would add is there are lots of younger, i.e. non-retired, expats in Mexico working for various international companies, large and small. In those cases the company assists with obtaining visas with permission to work.Sure, some do, if they have the correct visa status and INM permission, plus all the necessary state and federal documentation required for whatever they wish to do.
Others work online only, with no payment or customers in Mexico.
Requirements to start a business are best discussed with consular officials, but will also have a lot of similar requirements.
Work that would possibly take work away from Mexican is not likely to be approved.
Note that wages are a tiny fraction of those in the USA.....really tiny!
Basically, if you really need to work in order to live in Mexico, you probably are not going to qualify to live there. An expat in direct competition with a local may find more trouble than pleasure, and it can get pretty serious.
I'm personally shying away from Lake C, as I am worried about how cliquey I have heard it is.Your concerns about adjusting to retirement are not to be taken lightly. Especially if it's to be in a foreign country. For this reason I strongly recommend life in the Lake Chapala area.
The choices are too many to list and there is no excuse to be bored which probably drives many to return NOB.
I thought I had to have the "real Mexican experience" and moved first to a small town in Veracruz. I was able to enjoy my interests and more with only a handful of expats, but not my husband. You can only polish a Harley so much. I sometimes think if we had moved to Ajijic first he might have stayed.