The Tropical Fruit Forum

Clipping off the thorns does not harm a citrus tree.

Great example of what your writing about.

Your correct that Xie Shan is a great citrus.  It is one that every collection should have.

Pick citrus when it's full-colored, heavy for its size, SLIGHTLY soft or spongy when squeezed, and tastes sweet/flavorful, as citrus doesn't ripen further after picking.
From internet.

Very interesting.

Great looking fruit.  How did it taste?

Has the tree out grown the container? Also look to see if the the bark chips has disintegrated into a mesh.

You might also try hand pollinating the flowers with a little brush JUST as soon as the flowers open.

A citrus tree drops all its flowers before fruit develops due to stress from environmental factors like inconsistent watering (both over- and under-watering), temperature extremes (like a late spring frost), or nutrient deficiencies, especially a lack of potassium. Other causes include poor pollination, insufficient sunlight, or a young tree needing to focus on root development. The tree is essentially shedding its blossoms to conserve energy and resources when it is under stress or unable to support the development of all the fruit.

Taken from google

Looks like damage from insects.

I wonder if a red colored grapefruit would turn red in the UK?  The red color in grapefruit requires a LOT of heat during the fruit's growth.

While grafting healthy they green scions is is always first choice, I believe you could successfully
graft the light green scions you ask about.

Frequently ones finger to test for moisture can cause problems.  The top inch of soil frequently is dry, while the deeper soil still fully wet.  A better method of testing is by lifting the container.