i simply want to merge those 2 lists at their respective index but i have the feeling that the empty list is creating a mess
Is this what you’re looking to get?
Looks correct to me. You’re just inserting a list at the front of another list, not “joining” them. As Kulkul shows above, you just need to flatten your new sublists.
I finally managed to get it done using List@Level and keep the list structure