There is a lot to take here. It would be better if you always can simplify the problem by removing redundancy and isolating it where you can only re create the scenario you are trying to resolve. You dont have to use the exact data specially when its not worded in English as you can spend sometime trying to create a dummy data that resemble your structure which is easy to understand and readable by others. Keep in mind the more time you spend simplifying and articulating your problem the higher the chances it will get attention and the it will get resolved. Please keep that in mind when submitting other posts.

As far as your problem goes, it was hard for me to follow everything but something that might be causing the issue is  the spec for moving Tipo, Percentuale & Importo under the "ScontoMaggiorazione"  object in the first transformation as in the following:

...
"@": "FatturaElettronicaBody.DatiBeniServizi.DettaglioLinee[&1].ScontoMaggiorazione[&1].Tipo"
...

If you are trying to group those values under an array then use # instead of &1 , as follows:

 "@": "FatturaElettronicaBody.DatiBeniServizi.DettaglioLinee[&1].ScontoMaggiorazione[#].Tipo"

Using &1 ended up creating null object which probably caused  the generation of  "Importo" : "."  in later transformation.

If that helps please accept solution.