Is another expiation obligatory for someone who swore not to return to a sin, then returned to it, after having paid an expiation for a previous oath concerning the same sin? And does the simple fluid that exited after foreplay count as semen that necessitates expiation?

If you violate the second oath after having performed expiation for the first oath, then a second expiation is due upon you, by consensus of the jurists. Multiple expiations are only incurred if you violate the second oath before performing expiation for the first oath. Regardless of whether the fluid is madhiy (pre-seminal fluid) and not maniy (seminal fluid), expiation is incumbent upon you because you swore not to do two things: "that you would not practice it nor discharge maniy unjustly." The oath here is regarding two different things, indicated by the repetition of the negative particle, and expiation becomes incumbent by performing either one of them.