You can argue for changes while still recognizing that the current rules make it more optimal to do the things you argue against. There is no contradiction.

Libertarians aren't anarchists. They don't generally believe that there should be no government (although you can find people who believe just about anything). I don't think there's anything hypocritical about libertarians taking government contracts for defense. If they were taking government contracts or jobs around regulating markets or building public housing, then that would be closer to hypocritical, but I'm not sure that even qualifies. Hypocrisy isn't benefiting from something you disapprove of, it's engaging in it, so arguably, they'd have to be the ones issuing the laws they disapprove of for it to be hypocrisy.

Defense/military is one of the few government functions that libertarians endorse as totally necessary. To many, it's the only legitimate government function.

I don't think you've thought about this very hard.