If standing up a military is ethical in the first place, then as long as the recruiter provides correct and complete information to/from the applicant and does not apply any coercive threats/rewards to the potential recruit, everything is on the up and up (irrespective of whether the recruiter would be able to serve). The other items point out conditions where the likelihood of ethical lapses increase, but don't in themselves make the activity unethical.
Consider Unite States Army's oath of enlistment:
"I, _____, do solemnly swear (or affirm) that I will support and defend the
Constitution of the United States against all enemies, foreign and domestic;
that I will bear true faith and allegiance to the same; and that I will obey
the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God." (Title 10, US Code; Act of 5 May 1960 replacing the wording first adopted in 1789, with amendment effective 5 October 1962).
given that U.S. recruiting officers have made this oath themselves (since they're members of the military), the recruiting officers have an ethical obligation to perform their duties as ordered; this obligation is both personal and civic in nature. Similar oaths are used for declaring citizenship and various official government positions in the United States (might be relevant for sub-question 2); I assume that similar binding oaths are used in similar circumstances in other nations as well.
The primary professional ethics issues faced by recruiting officers are first dealing with legal issues of the applicants, second, not providing false information, and third not offering false inducements (or threats) to potential applicants, c.f. section 2 of these policies. The last two are similar to other "sales" types of positions, albeit with larger stakes. An example of an ethical failure of this type was an officer in 2005 promising the potential recruit that he/she would not be sent into a combat zone (Iraq) -- a promise that the recruiter could not ensure.
For question 2, for anyone whose made an oath similar to the quoted one above (note that this includes all naturalized U.S. citizens), then performing their civic duty by assisting with recruitment rather than "front line" military service seems like a good way to discharge the obligations that flow from that oath.
What I have in mind in the following situation: an individual has pledged their allegiance to a nation (not necessarily in a official/public way), is unable to directly serve in the military, and instead fulfills their pledge by assisting in recruiting.
For 3, I'm unaware of a theory where the level of payment in and of itself transforms an otherwise ethical act into an unethical one (or vice versa). That being said, higher commission levels would seem likely to increase the prevalence of the types of ethical issues that are specifically relevant for military recruiting.
The casualty rates are irrelevant per se; but again differing levels of risk would affect recruitment rates and thus put pressure on the officers to misrepresent the true casualty rate to potential recruits. To summarize, questions 3 and 4 don't in themselves automatically make the activity unethical, but they do create the conditions where ethical violations are more likely.
I've structured this discussion to emphasize the the role of oath-keeping, including oaths of allegiance to a government, that apply to this activity, at least within the United States. The ethical responsibility to keep ones oaths is pretty incontrovertible, and sufficient for the discussion in this answer. One could also draw on broader ideas of civic responsibility, but there is complications in doing so.
The tone of your question seems to imply that there should be issues with recruiting. I can only see this as a problem if the very fact of standing up a military is the ethical issue; if so that issue is prior to any of the detailed issues in this question.