You might want to re-read the Wikipedia page. It explicitly says that Voyager 1 saw the density of plasma around it increase by a factor of 40 as it crossed the heliopause. (For Voyager 2, it was a factor of 20, as I have posted elsewhere in this discussion.) It also explicitly says that the solar wind is stopped at the heliopause due to the pressure of the interstellar medium, which, last I checked, means the interstellar medium is interacting with the solar wind.
> conservation of momentum means the particles leaving the sun don’t stop rotating when they leave the sun - the sun is rotating
Sure, with a period of about 27 days. Go do the math and compare the tangential velocity that equates to with the tangential velocity required to orbit the Sun just above the Sun's surface.
As I understand it it’s where these particles reach equilibrium with the stellar medium. The sun is like a comet at a large enough scale, with a long tail of particles as it moves through the galaxy.
https://en.m.wikipedia.org/wiki/Heliosphere