That’s a hard one…
… I mean successfully booted and unbootable at the same time 
Some more information would be great, I don’t have a locked phone to reproduce this:
- I assume you already tried changing the slot to b (is that even possible while locked)
- I assume you already tried running
fastboot flashing unlock, Android docs only mention it shouldn’t work - What actually happens if you choose Recovery mode in the bootloader or use
fastboot reboot recovery
If all else fails, Emergency Download Mode (EDL) would be a way forward, but I haven’t found leaked firehose files for the SoC yet. So support are probably the only ones who can help you there.
