Are you serious?
To only drain a bit of fluid is like still swimming in the pool after it has been urinated in. Just taking a bit of water away and returning fresh water doesn't dilute the contamination. I think you are deluding yourself and any auto, "so called specialist" who maintains that idea is simply cutting corners and making more money doing part jobs more often. They mustn't understand CLEAN is best, not sure why. Very little oil is held within the cooler/heat exchanger at rad bottom, a small amount, there is not much room in there. Ever had one out of a radiator to actually see?
The shifting cannot change to be better than when new with fresh fluid. ie full flush infinitely better than piddling a bit now and then. You must have an incredible sense of detection to notice what a small amount of clean fluid might achieve.
By all means observe the temp and volume sizing if fiddling.
Do the so called techs at Isuzu really overfill the boxes? what evidence is known of it? the refill of any sort can be easily level checked by removing the rearmost plug in the bottom and when at correct temp any excess will flow out and correct level will occur. I think they know to do that.
Why operate in Farenheit when the world uses Celsius and the 40C to 45C is easily read by a digital thermometer.
I cannot see how the downshift for engine braking is affected by a bit of oil level difference, since ALL the retardation is being done in the torque converter when a ratio is selected. If a small bit of new fluid makes a difference, imagine what a fluid replace of ALL fluid will do!
What vapour is there to leak out of the rear of the box?
What auto box are you talking about, Don't Isuzi and Toyota both use AISIN boxes.
Be careful what oil you administer to the box, you speak of Castrol transmax and then say it is only a DX111 fluid. If I has the choice I would use the Brand name DX111 made by the manufacturer of DX111 long before ever putting Castrol into the auto.
Does your vehicle use a Synthetic auto fluid or a DX111fluid? Best to make sure. Many oil mixers make what is termed DX111 but genuine DX111 is what I would use as ALL others are made to a similar but perhaps not same spec. I thought but haven't checked, that Isuzu boxes use a synthetic fluid, may be wrong and haven't checked the specs either.
Do your own research and see if you have the correct fluid, and change the whole lot, which is what most people and motor shops WILL NOT DO. I look at it from the perspective of an A Grade Auto Engineer who has rebuilt quite a few electronic autos. Some because the owners fiddled with the fluids. Always opt for full change of CLEAN new fresh fluid. ie, Comprehensive insurance not 3rd party only.
PS, An Isuzu box will get to warranty time/distance without ever changing the fluid but shorter overall life.