Sounds like something is binding in the universal bar frame.  This is a semi-circle-shaped piece of metal behind the type bars, and when a type bar hits it, it is supposed to hit the escapement mechanism so it advances the carriage one space at a time.  You may have something dirty, something bent, or something out of adjustment.  Not being able to see the machine, the only thing I can tell you is to try to trace the movements of this bar frame, and see what it pushes, and where it starts to bind for whatever reason.  Good luck, and let me know how you come out.

You may also have an alignment problem.  It could be that once the carriage is folded down, whatever the universal bar frame is supposed to hit is either too high or too low.  Fold down the carriage to see where the levers all go and bring a type bar up to bring the bar frame out to see what it is nearest to, and see where the escapement mechanism is in relation to the bar frame.  That's the only thing I can think of without actually seeing where the two mechanisms land in relation to each other.  Good luck.

One more piece of information. Whenever a key is pressed (when the Corona is in folded open), including the space bar, the ribbon frame rises, the little plate with the ribbon color selector turns inwards, the universal frame bar moves to hit, but when you release the type bar (the carriage did not move one space ahead), the mechanism DOES NOT FALL BACK INTO ITS ORIGINAL POSTION. Only very slightly moving back. The letter bar falls back into place, the space bar rises up, but the central ribbon frame (where the type hits the paper) remains stuck in the "up" position and the plate color selector does not return either, remains inwards. You have to manually bring them back where they were. But only a VERY LITTLE effort, nudge, is enough to achieve this.
This phenomenon does not happen if the universal bar frame is not "engaged", the Corona is not fully unfolded, not resting on the bottom frame properly, the carriage is just hanging in the air, held by hand.  I wonder if this helps in identifying the problem.

I checked the two little hooks at either end. They seem undamaged, properly sprung. I can see that when the machine is folded open they engage with cogwheels under the ribbon spools, making them advance one slot each time a bar is hit. This part of the machine is working well.​The fact remains, however, that the hammer on the middle of the universal bar does not hit anything when a type bar is hit, literally nothing comes into contact with it (even though it travels the full way), so the carriage stays put. I will make a short low definition video of this by tomorrow to show. Hopefully the link will work here. Thanks.

I have noticed a further issue since posting the pictures.
​In addition to the bar hits not forwarding the carriage, depending how the escapement mechanism is positioned at the bottom,
- ​either the Backspace works well and the Carriage Release does not (it is stuck); or
- the Carriage Release works well and the Backspace does not (would not go all the way down).
There is no situation in which both the Carriage Release and the Backspace operate simultaneously.
There must be a logical explanation for these problems.