What if I accept an Offer and then find out, even after a hunk of optimizing, that it doesn't bring a profit or even loses money? Can I opt out at that point?

The best advice would be to do some research on profitable offers (or offers that have a potential of being profitable). The aim is not to abort an offer whenever you perceive that the campaign is not being profitable. Once you are confident in the offer, good faith says to stick it out and try the necessary means and suggestions (which you may get from us) to ensure that the campaign is successful. You can always transition over to local clients and take an affiliate offer that isn't profitable yet even, and make big bucks doing this. It's always an option.