1. Is obligatory service a violation of one's rights? why?
Yes. It's a form of indentured servitude and therefore violates people's natural right to liberty. I think America's constitutional law has it right when it demands that such violations be narrowly tailored to serve a compelling government interest. I don't see how obligatory service could pass this strict scrutiny, since America demonstrably can wage more wars than it should with the volunteer army it already has.
2. At what point (under what circumstances) is a draft necessary?
As a measure of last resort, if and when an all-voluntary army is demonstrably inadequate for defending the nation against foreign attacks. America's current dilemma, that the armed forces are are overextended because of America's wars of choice, doesn't nearly qualify.
3. In a time of national crisis and/or war, does the country's (govt) needs take precedence over the citizen's wants/rights?
I agree with the Declaration of Independence on this one: "Governments are instituted among men to secure these [inalienable] rights [that all humans are endowed with], deriving their just powers from the consent of the governed. If, in a time of crisis, a government no longer wants to be in the business of securing human rights, it needs to leave the business of governing altogether.
4. Your opinion!
In a word: nay.