Hello Howlin, my IL2 was previously patched with 408, and all the mods installed are working fine. I'had never installed packs till now and I know that I do that at my own risk...
As you suggested I repeated all the installation process but the Spit still doesn't work:
1 - deleted the \SpitfireMkXVI folder from C:\Programmi\Ubisoft\IL-2 Sturmovik 1946\MODS
2 - deleted the SpitfireMkXVI skin folders from IL-2 Sturmovik 1946\PaintSchemes\Skins
3 - deleted all entries in air.ini, plane_ru.properties and weapons_ru.properties
4 - downloaded again US_Weapons_AAA_UI12, unzipped the file, dropped the folder US_Weapons_AAA_UI12 in C:\Programmi\Ubisoft\IL-2 Sturmovik 1946\MODS and checked for it
5 - downloaded again AMT_SpitfireMkXVIe_V1-0.zip, unzipped and launched the installer, pointing to C:\Programmi\Ubisoft\IL-2 Sturmovik 1946
6 - copied and pasted the entries from "Readme SPIT16.txt" into air.ini, plane_ru.properties and weapons_ru.properties (at the end of these files).
I'll install other aircraft to test if I have some kind of button issue and after that I'll try to disable my mods to look for conflicts. If I'll find something I'll tell you.
Thank you to everyone for your kind support.