Wasn't Jeremiah a grouping in some contexts, as when it says "as was said by Jeremiah" and such a statement being in the Jeremiah grouping?
I've not heard of the DSS version of Jeremiah being significantly shorter before, so I'll have to look into that and get back to you. On the face of it, however, I'd have to ask, "How do they know?" Given the disintegrated nature of the DSS, how are they discerning that Jeremiah was actually shorter rather than just 1/7th of it was turned to dust?
Shalom