Serial protocol and connector choice for tethered firmware distribution project
Multiplexing the SPI bus is easily done if you select the correct types of switch parts for this. Do not tie the slave device select pin to GND at the slave. The slave select signal initial transition is the thing that provides the sync for the slave device to know when the boundary of the first byte of a SPI transaction is going to come. If you skip its usage you risk the high possibility that a slave gets out of sync with the master controller.

There are some connectors that are inexpensive, robust enough for this application and manufactured in huge volumes. One type that comes to my mind to suggest to you is the ubiquitous DSub-9 connector. It seems like it would be perfect for this application. Use 2 pins for the charging wire, 2 pins for the charging GND leaving 5 pins for your SPI interface. Instead of using a pair of 16AWG wires for the charging and GND you could go to a smaller gauge (18AWG?) and double to two pairs. This would map to the connector pinning better and lead to a more flexible cable.