Q1 Does the EntityManager of the surrounding transaction (here em1) see the returned entity as managed or detached?
A1 The returned entity is unknown to em1. See the entity as detached.
Q2 If the surrounding transaction commits, will there be a OptimisticLock exception, or is the changed version propagated to em1?
A2 There is no Exception. It looks like all changes are known to em1, because of the "call by reference".
Q2b Will the behaviour be different if the version filed is exposed to the entity (by annotation a field with @Version)?
A2b Don't know. My example had the version field exposed.