Again a brief recap: Nell learns that her formal "Farewell" will be "moderate" and will be carried out at the same time as the "Welcomes" for a young arsonist and her lover. At the very end of Nell's last letter, we learn that Nell has been charged with dispensing the Welcomes for the two young people. This was a "twist" that I never saw coming!
As for how Nell's Farewell compares with that postscript in both Nell in Bridewell and Lenchen in Zuchthause, the huge difference is the formal Farewell. In all of these, Nell gets a pardon and a release from the House of Correction. Only in Lenchens Abschied, however, is Nell subjected to that formal Farewell.