The correct key combo for recovery mode is Vol UP + power key. Please try that

Also good idea to remove any sd card from the device if you have such there. Unfortunate partition label could cause an issue..

I've updated my first post with hopefully enough info as to what I've tried and what does not do the trick Thanks for reminding me!

Sadly, what most likely happened, is a deleted/faulty SBL partition, this is the Qualcomm bootloader damaged, and only repairable by Qualcomm tools. The device boots to this mode when normal bootloader partition is damaged, thus the device can't boot.
So far none has figured out how to repair this, outside factory (except on XDA forum, for a few devices).
This happened for at least one user here, on J1 (search my answers)

Well if that really is the case - and by all means a quick google search pushed me into the XDA case too - then Qualcomm thus Jolla should have documentation on it, should they not? Maybe they can share what is needed - or tell us why we can't have it.

Thank you for this insight. I will wait for an official response from Jolla before any further tampering with my Jolla C in its current state.

Regards!

Hero.......
Aquafish is your way to go

MoritzJT, seems obvious, but have you let the battery to rest properly? In the early days of J1, I issued a halt from command line, and the phone was a brick. But battery out for 15 minutes and back in and then it started. But as I said, you probably have tried this already.

Thank's for the heads up, sadly that's also one of the things I had tried.

The ticket I opened with Jolla Care was closed and I have been redirected to developer-care@jolla.com - forwarding them my issue. They are yet to answer me.

I hope I'm not screwed here. Back in my Symbian days, bricking a device was nearly impossible - even dead flashing and correcting the partitions layout was easily possible, because we had all the tools necessary. With Jolla, Qualcomm and what not - this is an extremely sad situation. Dependency on the big manufacturers and THEIR rules makes me angry.

A little update - Jolla C does not come with any warranty according to Jolla. I'll have to pay for transport AND reparing. I'm waiting for Jolla to tell me how much it will cost to repair my device and decide whether to send it in.

Possibly if you can wait to the next Jolla event you might find some sailor willing to reflash it for you...

What kind of Jolla event would that be? I've never been to one. Do sailors themselves have access to the factory images?