You could try Dean at Small Car Services (
https://www.smallcarservices.co.uk/index.php/services/)
I can not guarantee that Dean can help but he may be able to provide additional leads.
He certainly has the offical Coupe Workshop Manuals (all updated etc.) as I gave him all my hardcopy manuals back in 2022! He also has/did have elctronic versions and also access to Fiat UK resources. He also is expererience in some Fiat gearbox rebuilds.
A lot depends on what you have actually done/damaged in your gearbox and that will require a strip down and inspection.
From what you are describing it looks/sounds like a selecter shaft issue. That could be the cars gear link controls (external to gearbox) or internslly to the gearbox.
As for UK Fiat resources then sadly if the Fiat is over 10 years old then support may be very limited if not out of this Universe