Why isn't it a distance squared relation?
There is a relatively sharp transition— the Heliopause— from solar wind dominating to interstellar “wind” (from other stars) dominating, which occurs where their respective radiation pressures are roughly equal:
https://en.wikipedia.org/wiki/Heliosphere#Additional_heliosp...
The specific dynamics of the cloud (e.g., its turbulence) are still largely unknown, though the fact that the direction of intersellar wind has shifted by several degrees over a few decades hints that it’s not totally calm.
I’m not sure about solar sails outside the heliosphere... that’s an interesting question. I’m not an expert, just a guy who likes reading about this stuff.