Assuming you want to keep your data … I don’t know, I didn’t encounter this problem myself yet.
I did a few slot switches myself successfully already when testing stuff, but I’m not affected by this current version 0.12 trouble, I simply (and luckily) wasn’t quick enough to install that version before it got retracted.
I see you asked the developers over there directly. If you have the time to let them answer and possibly figure something out, I would say this is your best shot.
In case you left your bootloader unlocked (I know, who even does that?), you could
fastboot boot the TWRP recovery and see whether it would still be able to backup your data partition and let you copy the Internal Storage via USB (TWRP doesn’t backup the Internal Storage, but it does MTP mode for a USB connection). Together with a fresh OS install there would be a chance for a successful restore.