(Not seeing double!)
Small news, but good news nonetheless!
That missing window regulator was missing out of error: the seller sent it over a few days after I mentioned it, and indeed, here it is. I can now say that I have a reliable source of (certain) new parts.
Also, given the dry weather of the day – and a little unexpected free time – I took advantage.
The passenger's door is ready for its 'transplant' though the driver's door asks for a couple extra steps beforehand: in the picture below, a plastic membrane is visible (which, mysteriously, was never fitted to the passenger's door).
Since it's glued on and isn't a factory fixture, it will be removed – its glue, "goo-goned."
The window trim clip and under-seat bumper are both now installed. It turns out that the Fiat bumper is shorter than the Zastava one (it never ends, does it?). Fortunately, ingenuous use of a washer made the heights line up: the driver's seat now sits solidly, like it's supposed to.
Another equally-dry day with some more free time will bear the replacement of the window regulators and the mounting of the new gutter trim.
Meanwhile, the search for the new fusebox continues...