No.
“Grabbing yourself some rank” is not acting under the Color of Law which is a legal requirement for the application of the De Facto Officer Doctrine. There is well defined process in law for establishing the legitimate military hierarchy of command.
Color of Law:
The appearance of a legal right.
The act of a state officer, regardless of whether or not the act is within the limits of his or her authority, is considered an act under color of law if the officer purports to be conducting himself or herself in the course of official duties.
Under the civil rights act of 1871 (42 U.S.C.A. Section 1983), color of law is synonymous with State Action, which is conduct by an officer that bears a sufficiently close nexus to a state so that the action is treated as though it is by the state.”
To have an analogous situation for the “game,” you would have to have announced that you intended to “grab some rank” a year and half before you showed up at the PX. Then you would have to compete against seven other members of your unit for the right to be in command. Then you would have to compete against a “rank grabber” from another unit; then win the approval of enough members of your branch of the service who cared to participate in the selection process, and then win at least 50% of the votes of the Joint Chiefs of Staff and withstand 202 courts martial hearings challenging your right to the position over a five year period without a single ruling going against you. Finally, you would need to be sworn in to your position by the Judge Advocate General for your branch of the military.
If you accomplished all that, then I would say “yes,” De Facto Offcer applies to your command and use of lethal force at the border.
Therefore the question is: would their actions be lawful or unlawful? (Legal or illegal? Legitimate or illegitimate?) -- Moreover would the federal government's judgements be in line with that?