Suppose sometimes I want the ZB mesh (& the complexity it forces), but other times I just want the simplicity of 802.15.4. Can I run an S2C as either? So turn off the ZB layer?
Sure; S1 could be used, but that is different HW which will upset some aged & well understood battery calculations.
Yes, S2C hardware can load with either 802.15.4 or Zigbee firmware using XCTU.
However, do note that you need to keep all nodes in network to same protocol i.e. either Zigbee or 802.15.4 at any instance. A network can’t be mix of both.
Connect Port X2 gateways already comes in Zigbee variant. You can straight away use it with Zigbee LT sensors creating a Coordinator-End Device configuration.