I looked at this code briefly. It is a very large program and I have very little time today.
Nowhere in MicroDexed.ino are Serial1, Serial2, Serial3, Serial4, Serial5 or Serial6 used. So I do not understand how this code could possibly attempt to communicate with an Arduino Uno using any of its 6 serial ports. Maybe the serial communication code is located in one of the other files? Sorry, I do not have time to read through a huge amount of code. I want to help you, but I have limited time.
Also unclear is which of the 6 serial ports you are using. The only info I see is "using a voltage divider of 1.66K ,1K between the TX and RX on teensy". But which TX & RX pins? What is the actual circuitry? No diagram, no photo, no clear description.
I also do not see the code used for Arduino Uno.
Could you create 2 very small and simple programs which do the board-to-board communication? Yes, this is extra work for you. But as you have shown, with a huge program for Teensy and no program of the Arduino Uno, and no photos or diagrams of how the hardware is actually connected, it would take a miracle (and many hours of work) for anyone to help.
Please, if you want help, show us how you connected these boards. Use photos. Then give us small but complete programs to run on BOTH boards. The code should be small and simple, showing only the communication problem. Anyone reading should be able to copy the code into 2 Arduino windows and upload to these 2 boards to recreate the problem.
I want to help you get this working, and I have both of these boards here and plenty of wires and resistors to quickly recreate the connection between them. But I can't spend a lot of time reading through a huge pile of code and guessing the code on the other board, not to mention guessing which pins you actually used.