Is the SDA and SCL on the left side of the Uno the same as the SDA and SCL on the right side ?
Why do you need those I2C multiplexers ?
Can you give a broader view ? What is your project about ? How long are the distances between the Arduino Uno boards ?
Maybe a RS-485 would be a better solution for you.
You need that WatchDog because there is something seriously wrong.
I can tell everything that is wrong, but it is easier to say that this is not going to work.
Even the pullup resistors are too low in value, you don't have a normal I2C bus anymore.