You already know, and all 3 technicians would tell you.
A trans with burnt fluid, when it was recently flushed,
needs an overhaul or a replacement. even add a cooler.
Cooked fluid can build up in passages and cause fluid
circuit blockage, then you start losing functions or have adverse operation. even trans stuck in wrong gear, or loss of ranges, engagement of t-case in wrong gear may be indicated. Reading up on the many websites can attest to this without getting out of your chair.
Tune issues that do not produce codes can still need repair,
but component testing must be performed on all the components.
That takes time, and costs money.
When you have burned or discolored fluid, you dont keep driving.
If a car's owner has been advised about his trans issues,
you dont keep testing. he's showing over 170K, is he going to fully invest?
You advise further that if he wants to fix it, he make
a decision about his budget choices.
Comments about "mechanics" trusting codes for diagnosis only?
It is designed into the process. It is an indicator. The trans electronics are involved as the known need of correction is inspected, the code is not corroberating, so, does he want to rebuild ?
But you have an expensive issue that takes precedence.
Every tech I know, would be waiting for authorization on the trans issue
rather than picking fly poop out of the ashtray.
You would already know to re-eval after the obvious.
but have to authorize unguided diagnostic but without safe road testing.?
UNH- uhh
Your "mechanics" are trying not to waste your time and money.
You need transwork.
Not some guesswork about driveability or runability.