So I've made what I think to be good progress in the last day.. My BT chipset is configured by default to use syncronous audio interface (sai) to send PCM data for HSP/HFP profiles and A2DP uses HCI to send audio data. After conversing with some others, it seems like PCM lines weren't configured in device tree/kernel/wherever so that's why no audio was being sent.
Once I configured BT chipset to send HSP/HFP audio over HCI interface, I did receive audio (although it was terrible, sounded like bad slow-mo editing).
So it looks like nothing was configured incorrectly from a PA or bluez perspective at least! Still got some work to do to make it usable.
Thanks a ton for your help
Still new to mailing lists; so I'll play it safe and send a separate email to linux-bluetooth with similar information :)