I must apologise, it won’t be the 3/4 OWC. I wasn’t thinking about it.
Reading it all again, it does seem that it is just the band not engaging in 2nd.
This could be either the 2nd apply pressure not getting to the band’s servo 2nd apply, or the valve body is not sending it there.
Guess it depends on if the box looks like it has been worked on or not.
If it hasn’t it may just be the B shift spool valve, that is not responding.
If it has been 'worked on', and they rebuilt the servo it may have a cut O ring, not assembled properly, or it could be anything.
If you have a known good, valve body and servo, it would be worth changing them over, as the fault has to be contained in one of them.
Harvey.