In relation to the use of a van or pick-up truck that does not have the required modifications, are these modifications permanent in relation to the registering of the vehicle?
These modifications must be permanent. If you use a van or light truck without the required modifications, then you must register it in the 'Commercial' registration class.
and how are the modifications of the collection trucks made?
You can register a pickup in the 'Passenger' class if it meets certain conditions, depending on its empty weight.