Good points but to me they are more suggestive of a great deal of confidence that Libby will not be found.
IMO he's not expecting to be caught but he's just raped someone. If there was any chance she was alive or her body left in the park he would be caught.
Once they'd tested her for rape his DNA would match loads of his other samples on the database. Even he must have realised police would take a rape more seriously than his other crimes and look for evidence. And with all that against as well it wouldn't look good.
I think he knew he was safe
He has a quick prowl round the student areas again after his third return to the park. It is more than possible that that masturbation was frustration at not finding a student so perhaps just filling a condom to leave on someones door to frighten them