What determined who would be a trainer?
Can't speak for your mission, but in Mexico we liked a good attitude, language proficiency, propensity for obedience, relatively decent numbers, and clean reputation (no issues, no rumors, no drama). If these missionaries got tired of training but weren't gonna be promoted, we'd reward them by assigning them weak companions—elders that needed baby sitting for mental issues, basically. Former trainers that could successfully handle two months of hell from companions with mental illness with a good attitude (and it was hell), were rewarded with either a promotion or several months in paradise (seriously awesome areas). Those who couldn't were still given rad companions and areas and perks for trying hard to make it work. Sometimes some cases are really rough. Headquarters approves a lot of seriously messed up dudes for missionary service that should never be anywhere near a mission, but I digress.
What determined the area you would be sent to?
A lot of it was arbitrary. But there were plenty of cases where, if a missionary was rocking it (like, not just numbers, but seriously good attitude, worked hard, traumatized by something in or out of the field, had plenty to complain about but didn't, etc.) we'd hook them up. Sister missionary making a companionship work, when it's no secret that she's been paired with a psycho-bitch drama-momma, we hooked you up.
Lost your steam? Bad attitude? Obedience issues and general laziness? We called you in to be a secretary. Worse? Like, you're a dangerous and manipulative personality type probably getting away with murder? Pres. would call you to be a traveling AP, like an aux AP. You'd spend most of your week splitting with the APs so we could figure your shit out.
Just finished a stint in paradise? Almost guaranteed you'd be in the most putrid parts of the mission afterward. Really bad places.
Did your parents just get your letter with that posed pic of you in slumville with a shit-eating grin on your face and your 20 page letter about how you're finding dead bodies in the canal and you're so blessed to be an American helping the poor? First, fuck you and your privilege. Second, they called and now we're moving you to paradise for a month so that your letters aren't so self-righteous.
How much of the decision-making process for transfers did the mission president take upon himself?
In our mission, about 10%—sometimes more or less. Mission politics are terrible. We had two different presidents, and life under one was so traumatizing that we spent a lot of our time cleaning up unnecessary drama in his wake. Dude's a GA, now.