I can take a shot at both.

Re: question 1. Once someone volunteers for that professional force and signs the contract without coercion, they lose all right to object to being told to do what they voluntarily took on. I think that means if you sign on, you're stuck. Don't want to be stuck -- seek other employment. Because it's a job, it's a trade, not a profession. Did I mention that entrance is not mandatory? Since it's not, the old saw 'be careful what you want, you may get it' applies.

Yes, draft or conscription changes that rule and conscientious objection is permissable -- probably should be encouraged...

On question 2, those engaged in the trade of soldiering have taken someone's Shilling, as it were, therefor they have an obligation to do what they're told. It as they say, goes with the territory. They do have the autonomy -- and IMO an obligation -- to exercise their own expert judgment in refraining from doing harm to an extent in executing the missions given as they see fit. They do not have the right to decline missions but have a responsibility to attempt to structure missions to best accomplish them at the lowest possible cost to own nation and force. If given a mission they do not believe is lawful or that is consistent with their values they may resign if possible or take the punishment prescribed for failure to follow orders or violation of their contract. Hopefully without whining about it in either case.

One always has choices.