I'm not sure it was really decided

I guess not? Same as a converted convertible that used to be a coupe, 'class' should point to what it is now or to how it came out of the factory first time..?

OK
For the one switched to Diesel I wonder if we should still list it as trolley-bus or not
https://imcdb.org/vehicle.php?id=703596

A small "mass" change
These should be 1938 110 AF TIBB Machi instead of simply "Trolley-bus". Last time I went with what was 'established' by previous posters but they should get their proper model type I think.
https://imcdb.org/vehicles.php?make=Alfa+Romeo&model=Trolley-bus+TIBB&modelMatch=1&modelInclModel=on

There's also these two
http://www.imcdb.org/vehicle.php?id=897390
http://www.imcdb.org/vehicle.php?id=925114

and this perhaps just the code [T10]
http://www.imcdb.org/vehicle.php?id=316097

I also added the T10 chassis code to those which didn't yet have it

Back then we simplified the codes of the VWs when we introduced that field on the site, but if needed maybe we could change to make the closer to reality

How about.. adding Autotutto to these so they're lumped together with the other Romeo Autotutto's (it was the official name anyway)
http://www.imcdb.org/vehicle.php?id=1155321
http://www.imcdb.org/vehicle.php?id=1247782
http://www.imcdb.org/vehicle.php?id=1400316
http://www.imcdb.org/vehicle.php?id=1566574
http://www.imcdb.org/vehicle.php?id=299567
http://www.imcdb.org/vehicle.php?id=614494

..and deleting the hyphen for these so they become LM002
https://www.imcdb.org/search.php?resultsStyle=asList&sortBy=0&make=Lamborghini&model=- LM-002 ..or LM004 for this www.imcdb.org/vehicle.php?id=55533

..and finally TIBB is not a name but initials for Tecnomasio Italiano Brown Boveri so it's better these change from Tibb http://www.imcdb.org/search.php?resultsStyle=asList&sortBy=0&make=Alfa+Romeo&model=- 140&modelMatch=1&modelInclModel=on (only the ones that already have this description that is!)

Is there any reason why first generation VW Jettas are listed as "Typ 17"?

They were "Typ 16" for as long as I can remember. I mean I know they are based on the Typ 17 Golf but they had their own code.

Same question for the Karmann-built Beetle Cabriolet/Convertibles which were "Typ 15"?