It was working for updating the UnitPrice upon a NominalAmount Change but not when doing a similar things for Volume.
Now removed everything. Moved back fields to Double (tried Int previously but there was no way it could work).
and it works smoothly with standard Property definition.
On your point about ugly UI, if fields are editable or not, it does not change much. Some industries have their own way of doing things because every client wants something different and unique which requires specific implementations of custom machinery at the plant level and the sales process particular. Take a look at the subtle difference between a water bottle cap and another. Never the same from a brand to the next.
Anyway, thank you for the assistance