(logic level) Gating the output of a 900hz (5v p2p) oscillator
I think I'll just answer b):


  is there a different approach for generating a logic enabled 900hz audio tone ? (…for fastish logic switching from the mcu)


You already have an MCU. Practically all MCUs I know contain at least one, typically multiple independent, PWM units. These are designed to give adjustable-frequency, adjustable-duty-cycle rectangular waves.

In other words: Drop the damn 555 and just use either your existing MCU to generate a 900Hz tone, or replace the 555 with a really cheap MCU, and low-pass filter appropriately to convert square wave to sine wave.

Note that 900Hz is really slow, from an MCU's perspective. You might, especially if you've got other timers/counters to spare, very simply do this in MCU software. The potential jitter won't matter, as it would be a high-frequency component to be filtered away, anyway.