Aaaand it worked the way expected, map is happy with tilesets B and C instead of the rebellious A !!
Phew… I can breathe now 
Yes, it was a simple scenario I can see that ! Trouble understanding comes mainly from my lack of knowledge about Tiled itself AND my interpretation of what the script does
Thank you really for this script and your guidance 
I really didn’t want to place tiles by hand again, you saved my sanity !