That alkaline limestone soil is likely going to leach into your more acidic loamy soil over time.
Also, the roots of that mangosteen are eventually going to expand out in all directions and reach the limestone.
Perhaps planting the mangosteen on a slightly raised mound over the hole? And maybe surround the edge of the hole with a soil partioner? (around the sides, not the bottom)
I'd make sure the hole is at least 30 inches wide and 30 inches deep (preferrably 36 deep if possible). Next you may wish to sprinkle some alum (garden store for lowering pH) at the very bottom of the hole before you fill it in, so if for some reason in the future the ground does become completely saturated with water, the alum will leach in with the limestone and you won't get a net alkalinity in the soil. Also I would mix in a lot of decaying compost into the soil, possibly also a little peat moss, because that can help slowly release some acidity as it breaks down.
Maybe plant a banana nearby the mangosteen to help provide some shade. Then you can always hack the banana back later if the mangosteen gets really big.
The hole is roughly 43 inches diameter by 2 feet deep.
I know it's really difficult digging down in that ground, but I'm thinking 2 feet is not going to be deep enough.
First, the roots are going to want to go down much more than that after 2 years. Second, you want a nice buffer layer so if any limestone alkalinity starts leaching in, it's not going to immediately move into the layer where the roots all are.
Keep in mind there have been many attempts with mangosteen in Southern Florda and most of them have failed, likely due to the limestone soil. You want to go to greater lengths than the average person would have been likely to go to.
How should I line it? Should I leave holes for drainage?
Around the sides, not the bottom.
Maybe add a layer of alum and/or peat moss at the bottom.
Also, any advice on soil composition is appreciated. "deep rich organic soil, especially sandy loam or laterite" is recommended on the hort.purdue.edu site. How might I achieve such soil?
Get the more expensive potting quality soil, maybe something organic.
Some of the cheaper landscape soils are lighter weight, have a lot of wood chips in them, and aren't really rich or good at holding moisture. Don't get those.
Maybe think about starting a compost pile.
I would mix in 30-40% compost with a high quality dark black potting soil.