after i installed the mod i can backstab with "most weapons", but not all, i have not tested every single item in the game however i have found out why some items are incapable of backstabbing eventhough it should be possible with the mod.
After some toying with the infinity editor i found that weapons with tag unable to be used by "Any multi/dual or single class with Thief", does not allow backstab.
this in particular example with carsomyr, it is still labled as paladin only, so it has unable to be used by flag ticked on for pretty much every single class out there, and this made backstab impossible apparantly.
so i tested by editting the restriction, making thief/clericthief/magethief/fighterthief able to use it straightaway solved the problem.
im not sure how many other weapons there are with similar restrictions, i expect all of them will fail when tried to backstab with them.