Mic Preamp with unshielded input transformers.

[abbey road d enfer]

I wonder if using a differential receiver chip would be preferable to the tiny unshielded input Tx that is picking up hum?

These differential receiver chips are not optimized for mic level, too noisy.

Also for a barebones preamp I don't think the 16 overvoltage protection diodes on the output are necessary?

Certainly not necessary, but they offer extremely cheap protection for these expensive chips.