I upgraded the firmware to v5.4.5 but hacks stopped working ... even if this time I managed to launch Collections Manager and Kual.
But no firmware downgrade possible!
Still searching in the forum, I found this post of NiLuJe, MobileRead Package Installer:
https://www.mobileread.com/forums/sho...d.php?t=251143
So I installed the latest official version of the firmware Kindle PW2, v5.6.1.0.4, then MobileRead Package, and finally Hacks (Python v0.11.N, MKK v20141129, Linkss v0.21.N, Fonts v0.10.N, Kual v2.5.16 and LibrarianSync v1.9) but no Jailbreak is still necessary!
And it works!
Except Collections Manager which does not work in V5.6.1.0.4 and I replaced it by LibrarianSync.