The Tropical Fruit Forum

The tase is sweet and sour, I find it somehow unique.

Probably reminiscent of Satsuma, I would imagine, though more sour and like lemonade.

Add sulfur to correct ph, yearly

Sulfur (we're talking about elemental sulfur, yellow, not sulphates) is good at keeping the pH down over time but I would worry about sulfur (again talking about elemental yellow) not being good for the roots (especially since mangosteen roots are notoriously sensitive). There is a chemical reaction in alkaline conditions leading to sulfites or thiosulfites.
It also might impart a less than pleasant flavor once the mangosteen does eventually fruit.

I know Meyer lemons can develop an off flavor if you wait too long to pick them.

Lots of things can lead to more watery insipid flavor in mandarins. For some varieties, it takes several years to develop decent flavor, and many more years for that flavor to slowly improve.

Dancy is supposedly supposed to be the gold standard for mandarin flavor (or so I have read), but I personally do not care for that type of flavor myself.
(I am wondering, are you sure it's not just personal preference and mandarin variety?)

Sometimes too much watering while the fruits are developing can lead to more washed out flavor.

Also sometimes climate can make a difference with some citrus.

Seattle and Paris have roughly the same climate conditions, with the hermetic cover in a sunny spot  shown for Bloomsweet the death of the plant is guaranteed.

Socal2warm I would not cover any of my citrus under a plastic cover for long.

The cover does have a permeable fabric mesh at the top to vent off extra heat. Although I would not be that concerned even if the plant was completely surrounded in the plastic. I'm fairly sure the plant isn't going to overheat during the Winter here, the temperatures stay pretty cool.

I took a measurement and the temperature inside the cover is only 6 degrees F (that's 3.3 degrees C) warmer than the temperature read right outside the cover on a sunny day.

And I'm sure our winter sun is much weaker than yours and its colder here too.

I believe the location here is comparable to the far South of Switzerland, at least during the Winter.
In about late November the climate transitions from a cooler Mediterranean-like climate to a wet temperate rainforest.

I am at the same latitude as you. Just not in the mountains with elevation. Although there is often very cool air blowing in from the Pacific Ocean, that I think approximates a high elevation level, at least more than half the time in the Winter.

Very remarkable that you are able to grow Keraji in Lucerne, although not surprising they haven't seemed to ripen all the way.

I'm sure there must be many culinary uses for green Keraji fruits even if they haven't rippened all the way.

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.

Bloomsweet with a protective plant cover over it, up against a South-facing wall in a sunny warm spot:

Ten Degree Tangerine

today

Here's a picture MarcinR (from Poland) posted in another forum of the results of a Triploid Reale x N1Tri cross

http://www.subtropicke-rostliny.cz/forum/viewtopic.php?f=22&t=475&start=60 (picture posted April 8, 2018 )

(I believe "Reale" is the variety 'Monreal' clementine)

Which is in climate zone 8a.
sure the long hot humid Summers help.

You're right, your mangosteen doesn't look all that absolutely healthy and pristine. Maybe I shouldn't have planted mine in an open area.

This is in zone 8, and I don't think any of this would be worth attempting if it was even in zone 7b.
I have read a few reports of non-hardy gardenia varieties occasionally being able to survive around the sound in this region (which seems very encouraging, if true).

As for fruiting, I'm not truly sure if that will happen. Unlike much of the rest of the PNW region further North along the sound, we do get some heat here in the Summer (although it's not a long Summer, and the nights can often still get pretty cool).
The Satsuma is in a cold frame enclosure outside, that's not going to be opened up until April, so surely that's going to be acting as a greenhouse of sorts, giving it the extra heat it needs (at least on clear sunny days).
It's also important to take into consideration that citrus grown from seed (not on rootstock) will be less precocious and take much longer until it begins fruiting, even though I think own-root citrus are more vigorous and hardy in this zone 8 climate. This combined with the shorter growing season and cooler Northerly climate could mean that it could take a very long time before a citrus tree begins fruiting, and the tree might be quite big by the time that happens. That's why I wouldn't necessarily give up on a seed-grown tree just because it's been growing for 10 years.

One common cause of delayed graft failure, it is believed, is differences in growth rate between the rootstock and scion.
Some varieties grow at different rates than others.