Alright gentlmen... Let me end your debate on what military roll Leia has in the Rebel Alliance right now. Princess Leia definitely does hold military rank, HOWEVER, the royal title of "Princess" out ranks any title in the military so that doesn't matter. As you see from the picture above Leia has a rank badge on her left breast pocket. She IS military... that much is certain.
As to what level of military rank... that is very debatable because it's never mentioned but I think it's akin to how the President of the United States is "Commander and Chief" but tactics wise and military doctrine he doesn't possess those skill sets outright and would need to be briefed and advised by those who are career military leaders ie General Riekian. Leia I'd say is second in command of the Alliance armed forces only behind Mon Mothma with the available information I have. Therefore technically, I'd say she out ranks the senior officer General Riekian at Echo Base.
As whether she is qualified to give a basic military briefing... This is a woman who was elected to the senate at age 18 and thus speaking in high pressure situations comes naturally. Militarily speaking, I'm pretty sure in a off screen military briefings they had layed out the contingencies for evacuation along with plans of action even prior to Riekian even giving the official order to "begin the evacuation:. Thus relaying the basic jist of the evacuation plan to a bunch of snub fighter jockeys is not a problem from someone of Leia's caliber especially if they've discussed such a plan as a preconditioned tactic.
The Rebel Alliance always prepares their bases as mobile units along with pre-planned evacuation procedures. Such things are generally on a need to know bases restricted to high ranking officers and high ranking leaders of the Alliance. Otherwise, you'd just have a chaotic bunch of guys with guns cramming whatever into ships without any coodination at all.