Worf: "according to my calculations, a solar probe launched from either the Klingon ship or the planet's surface will take eleven seconds to reach the sun."
even if they fitted the saucer with those "sustainer coils" that they apparently put inside photon torpedoes, so they could release the saucer at a low warp speed and it would keep going.
I never have bought into the idea that torpedoes only have a combination of a sublight engine and something referred to as a "warp sustainer."
The distance between the Earth and our sun is about five hundred light seconds. Worf in GEN said that a solar probe (admittedly not a torpedo) would travel a not dissimilar distance, from basically a standing start, in eleven seconds. Meaning the probe had to be capable of achieving warp on it's own, it clearly possessed a independent warp drive.
Again a probe and a torpedo are not the same thing, however they are about the same size, if a small warp drive were available for one, why not the other?
so they could release the saucer at a low warp speed and it would keep going.
And if they were unable to release the saucer at warp (for whatever reason), it would be "trapped" at sublight speeds.
Interstellar space is a big place, the saucer would have propulsion suited for travel between planets, not between stars.