The plan is to make an analog VCO centered around 455kHz. Then use an ATMEGA or M0 board to control the VCO using an analog output (tuning voltage). Maybe measure the VCO frequency using a timer peripheral to close the loop.
This project is in several parts. I'll add to the project logs as different circuits are evaluated.
I'm not the first to get confused about variable capacitance diodes. This experimenter seems to have solved some of the issues: https://sites.google.com/site/linuxdigitallab/low-noise-crystal-experiment/diff-neg-vco-1mhz-30mzh