Abracon RTCC - no startup oscillation, no SPI
The solution was to remove the AB1815, clean up the PCB, very carefully re-apply a little solder paste, and reflow a new AB1815.

Upon examining the original AB1815, it looks as though pins 12 (nCE) and 16 (XI) did not have the most reliable-looking solder bond. Rather than try reflowing it again, I decided to try another one.

I can't see much on the XI and XO pins, but it is drawing about 0.02µA (20nA) quiescent at idle and SPI commands are working now. Definitely going to leave non-populated load cap footprints on the PCB for the resonator just in case it is temperamental. Thanks everyone!

MOSI and MISO: Sent 0x10 (Read Control1 Reg), returned 0x13 (expected), etc.