Thanks Alison, that's very helpful!
Since I'll definitely be using this transceiver outside during the winter I'm tempted to stick with a 4S configuration for the sake of not getting caught short on power in the event that I can't keep the batteries warm overnight (this has happened to me before: https://summitsandradios.wordpress.com/2018/04/11/return-to-garibaldi-ve7-sl-009/). Instead of shoving the full 16.8v into it, however, given your advice it seems prudent to leave the protection diode in place and maybe use a regulator, or even a second diode, to protect the radio. Arguably I'll only need an hour or two of operating time out of it, facilitated greatly by the 50mA receive current, so maybe I can get by with 60% of a 3S battery pack. I'll have to do some testing on this. Maybe there's a 'winterized' configuration that I could use for half the year.