How big are the fruits?

They are probably not under ideal conditions, so the fruits around here are around 7-8 cm long and 3-4 cm wide.

The seeds appear to make up most of the volume – is that consistently the case?

Yup, it's almost all seed, the pulp is at most 0.5 cm layer around the seed. All fruits I've seen look like this

How would you describe the flavour of the fruit when the astringency is gone?

I'll replicate another comment I made: I honestly don’t know how to describe it, it seems to have quite a unique taste. It’s creamy, but drier and firmer than mango. I would also say it’s less sweet than most mangoes. It has a really soft flavor, that to me at least, is completely different from mango, more like a floral flavor to it

Are there any distinguishing features of the leaves or flowers (shape, fine hairs, etc)?

The underside of the leaves has a velvety layer that if you rub your fingers on it, it comes out quite easily. The upperside is smooth, glossy and dark green

I took another picture, but these leaves are younger, so they have a lighter green to them and they are smaller. The leaves usually have around 15 cm

The flowers are raceme, with white or cream colors, see: