So I've given up and finally conceded defeat - it still has me baffled, and completely beaten.
It's a mystery that will follow me to my grave - and it feels like it's almost driven me to a premature grave!
I have tried absolutely everything. I've taken both the Rexina and the Condor completely apart again - several times - and tried various components from one machine on the other. I tried manipulating the space bar, which remains too high and too close to the bottom bank of keys. I've taken the boots off the Rexina, cut them in half, replaced the bottom half, then the top half (above the base plate, under the mainframe), then replaced the boots in full again. In the end, the boots came off completely. Screwed into position on the base plate, with boots, the machine simply does not work.
A combination of no boots, no base plate screws, the mainframe sitting dead flat on the base plate and pushed out of position and right to the back of the outer casing, and a manipulation of the ribbon spools cover: all this put together has worked. But then that's a typewriter not properly reassembled. So I lose!
The ribbon spools cover was, I found, very subtlety different on the Rexina to that on the Condor. On the Condor, the centre line of the cut-out was noticeably straighter and flatter - it's hard to tell from the above image, but there was nonetheless a pronounced difference.
What I did was subject the Rexina cover to boiling hot water and gently manipulated its drooping centre lip back up to about the same level as the one on the Condor. Still, even this didn't work until I removed the boots altogether.
I must thank all those people who messaged me with suggestions:
RICHARD: It's such fun to share intercontinental mysteries ... I just checked my Alpina Baby and saw that, although it has those metal supports above the segment, the plastic ribbon cover doesn't even touch them. They are there in case of emergencies, I suppose.
I wonder whether the two rubber grommets that the cover's prongs fit into have deteriorated and shrunken so that the prongs are fitting too far into the body of the typewriter and the cover is coming down too low.
The metal supports touched the cover edges on all configurations, so I think this probably does help on the Condor.
The grommets were my first suspects. One was missing when I first unpacked the machine, but I found it inside the workings when I took the Rexina apart. It was slightly worn, but not badly enough to make a difference, I don't think.I turned it over, anyway, to provide a firmer top part.
SCOTT: This unit would have been functioning when it went on sale ... To that end, I'd look at the ribbon cover first. The tolerances are so fine on such typewriter designs, that even half a millimetre of warping would potentially cause issues.
Those clips. Have you tried putting them on the other unit?
Yes, the ribbon cover was minutely, fractionally different, with a more pronounced drooping lip in the centre of the cut-out on the Rexina. It may well have been warped at some time. I can't even say for sure whether this machine ever did work properly.
I switched clips and covers, but to no avail.
PETER: This is a great example of the entertainment value of even problematic typewriters, and the justification, one might say, for having bot variety and duplicates. Is there any place on the underside of the cover showing abrasion?
No sign of abrasion. I obviously didn't try to type with it on, apart from some gentle touches of the keys to check the clearance, and I doubt the previous owner had done any different. Still, that cut-out part of the cover was perceptibly different from the Condor's.
TYPEBARHEAD: ... my wild conjecture would be that perhaps the cover is not the original. Is it possible someone found a similar one of another rebrand that they thought fit and replaced it?
This does seem feasible.
Also thanks to Georg and Florean ...