Yes, the trombone solo does go on and on. But the latest scholarship as of 1994, on which Christoph Wolff's book about the Requiem is based, attributes the beginning of the solo to Mozart but its continuation to Süssmayr.
Christoph Wolff wrote:The most crass example is perhaps in the 'Tuba mirum,'.. where Mozart uses the trombone to represent the sound of the last trumpet (bars 1-18). The entry of a string accompaniment in bar 5 is an intelligent and skillfully executed idea (taken from Eybler), but Süssmayr then finds himself obliged to continue the trombone solo from bar 24. Without reflection, he follows Mozart's model (trombone solo), heedless of the change taking place in the character of the music, let alone the nature of the text in the fifth stanza of the Sequence ('Liber scriptus').
Wolff's "Mozart's Requiem," not only a detailed and engrossing study of the music but also including the full score, is well worth having by those who care about this piece. Actually, Wolff provides two scores - one of the Mozart fragment, leaving out whatever he didn't write down himself, and the other of Süssmayr's additions. Published by the University of California Press, in paperback.
Of the several completions of the score by other hands, the most persuasive is Robert Levin's. He says that the Süssmayr completion should be respected except where it is obviously defective or otherwise un-Mozartean, as Süssmayr was Mozart's assistant at the time, may have received some instruction from Mozart on completing the work (there's no evidence to support this), and may have had written sketches to work from that haven't survived. Levin retains the trombone part as we have it to the end of the bass solo, then drops it. From several footnotes in Wolff's book it's clear that he consulted with Levin - both were on the Harvard faculty then. Indeed, he once called Levin "the greatest Mozartean since Mozart," and that's not hype.
Incidentally, I've come across a fine recording of the Requiem (Levin version) that I didn't know, conducted by Charles Mackerras. Here it is: