I use several quite different routes and rat-runs ... there is irregularity within the regular activity ... Neither ... clearly gives that difference without some context.

I think the language is filled with usages that demand some context to understand, as perusal of almost any topic of discussion here can attest. I go back and forth to work. That I do so by different routes does not change in my mind the fact that I would use back and forth and not to and fro to describe what I do. I think it's more a matter of the end points being constant than the pathway.