Some preliminary testing* with the dev console suggests:
- Both the initial shield bump attack and the follow-up stoneshield bump can trigger flexible combat.
- Flexible Combat definitely does not trigger shield bashes.
- Attacks with non-shield mainhand weapons do trigger shield bashes.
- With a shield in the offhand and nothing in the mainhand, the player performs shield bashes with the offhand. This will trigger FC, but not additional offhand shield bashes. The player does not perform unarmed attacks with their mainhand, only as a result of FC procs.