Hi Paul - so do you think that the tri-state-buffer is the solution
It certainly looks that way.
I confirmed by hardware testing with resistors that the MISO pin was driven while CS was not asserted. When a chip tri-states the MISO pin, the resistors should take the pin to a voltage half way between GND and VCC. The voltage remained 3.3V (actually, a little less, since that chip doesn't drive very hard, but clearly still driving the line). You can pretty easily duplicate this test by just connecting two 10K resistors to MISO, and measure the signal with a scope or even a voltmeter when the display isn't being updated.
and will solve all the conflict's I'm having?
Yes, I believe it will. 2 displays that didn't work together, and which I confirmed were conflicting on MISO, clearly do work with the 74HC125 buffer added.
When the 3rd display arrives, probably within the next 16 hours, I'll hook it up and try your 3-display test with the SD card. I'm only going to run the test on Teensy 3.1. If that works, and I'm pretty confident it will, I'm going to consider this fully "resolved".
I sincerely hope Adafruit documents this limitation on their website and eventually makes a new version with the buffer and 2 pullup resistors. But that's really up to Adafruit. I have no control over what they do. I've done pretty much all I can here.