aren't retests USUALLY done in order to confirm an initial positive result?
Nope. They use the same sample to perform the confirmation test.

My question is this, although I would not expect a definite answer: Is 5 months abstinence sufficient? Well, at least to be in the clear for a urinalysis?
Usually

Edit: There actually had been a few cases in which an individual had still tested dirty after more than 100 days of abstination. This is probably the most recent thread: http://boards.cannabis.com/drug-test...a-shocked.html