2) I replaced the default RestEasy jar to my RestEasy jar (that is on the same version). Everything worked fine.
I don't see how the same jar ( == same version) can change things?
3) I've also tried to use the jboss-scanning.xml but as I understood it only works for intern app scanning.
jboss-scanning.xml limits what we scan wrt resources; e.g.
* annotations; @EJB, @Stateless, @Entity, ...
* config files; .hbm.xml, ...
Why exactly you need it here?
How can I do it? Can I declare what kind of jBoss jars I would like to load for my app?
There is plenty forum discussions on this topic, a few just recently.