Correct me if I'm wrong, but Jolla's recovery kernel only relies on its embed initrd ramdisk (that was certainly the case on Jolla 1)

It doesn't rely on data nor recovery partitions. You could nuke them and the recovery will still boot.
(But you won't be able to use the "factory reset" entry from the telnet menu. You'll have to drop into a shell a fix by hand).

So you could OFF ; Volume-key Down ; USB cable :
then OFF ; Volume-key Up ; USB cable for recovery ; follow on-screen instruction to telnet.

Or more directly :
to straight load and boot it without even writing it to flash.

I haven't had time to investigate yet, but I strongly suspect that the bootloader itself isn't stored on the eMMC flash.
So fastboot mode should work even without it, and fastboot boot should be entirely be doable in-RAM.
Thus this last option should still work even if you completely **** up the whole GPT partition and need to re-partition manually.
(But this isn't a guarantee. Maybe the bootloader needs to do something silly, like checking a checksum or a signature in order to decide if it needs to display a warning, and crashes in case of corrupt partitions)