[Spaceships] Interstellar Magsail Deceleration

[Ulzgoroth]

There's a fault in your approach. Magsails can be used to accelerate on solar wind, but that's not the basis of their 'parachute' effect. In interstellar space the solar wind is negligible (in fact, I think it essentially cuts off at a distance from the sun). But there's still interstellar medium, and in quite a lot of space that medium is ionized. Thus a craft moving through the ISM and c-fractional velocities naturally sees a headwind of charged particles that a magsail can use to break.