Your question is probably better asked over on the IronJacamar forums. From what I can recall, the XA connection is enlisted for you by iron jacamar when you request a connection within the scope of a transaction.
1. it would be an error to use an XA connection outside the scope of a transaction, so I would say, "no" to using it after the commit
2. If you call getConnection outside the scope of transaction I would say you will get a standard connection rather than an XA one, it does depend on the capabilities of the underlying connection manager, in AS7 case: IronJacamar.