I can’t help, but you could try to book via tickets.oebb.at  (one way ticket - at 1x adult choose discount Interrail/Eurail Global Pass) or at the Trenitalia Ticket Office at Paris Gare de Lyon. Maybe also SNCF can sell Reservations for Italy (2019 it was possible, now I don’t know)

Should be fairly easy to avoid treni with RES-its a kind of roundabout route and needs changes anyway (Firenze SMN and or/ Bologna)

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