Hi Sotirios and Guillermo,
I just finished my custom module. I used finally the entity_presave hook where I included my checks and it works nice and smooth. I first started creating a library on top of the override theme where to put js code but didn’t like it. I think creating a module and using the hook is a better approach and more Drupal oriented.
For anyone trying to deal with rules (both rules and business rules modules), in my experience, they just work with very simple rules.
The business rules module is pretty nice, having more advanced options but still has some bugs (some of them waiting for a patch for almost 1 year).
Thanks