I understand that the merge() method is to be used when reattaching detached instances, but what will happen in the following cases when working with persistent instances?

createEntityManager
find an ITEM
(the EntityManager doesn't get closed)
change some properties on ITEM
merge
commit
close EntityManager

OR

createEntityManager
create a new ITEM instance
(persist doesn't get called and the EntityManager doesn't get closed)
merge
commit
close EntityManager

1. merge is a no op and is useless, the item is already associated
2. merge will trigers the INSERT of your item

Note that merge does not reattach, it merges
PerssitentCopy = em.merge(detachedCopy);

Is there an EntityManager function that just reattaches a detached object to the session without doing all the checks/updates of merge()?

no but session.lock(, LockMode.NONE) does what you want... I think