Problem solved!

I contacted the guy with the ECU and EZK combo on a 245, but he wasn't willing to part with them. Originally he had intended to strip the 245 and sell off the parts but now had reconsidered and decided to sell it on in working order.

So, I had another look at the pinout diagram and noticed that pin 18 is pulled to ground when the injectors are supposed to work. The symptoms with my dud -949 seemed to point to a burnt through driver transistor as they were continually pulled to ground. Some tracing later and sure enough, T402 measured quite low resistance between C and E. Here's where my 19 years in the electronics industry was usueful.

That transistor was an ON823. Never heard of it, couldn't find much info other than that the bog standard TIP-122 Darlington transistor is a suitable replacement.
The only shop in town left selling electronics components had them in stock, so an hour later I was back home and replacing the defective T402.

When all was finished I connected the bare board temporarily on the connector in the footwell and tried control function 3 again. SUCCESS! The injectors clicked!
And of course now the engine would start just fine.

I decided to replace the main 470uF/40V capacitor as well just as a preventive measure as these things usually don't age all that well, losing the electrolyte and with it capacity over time.

While I was at it, I opened the -561 up again for a side by side comparision.
The designs are not identical, suggesting that the -949 is a newer one.
Date codes on the components suggest that these boards were manufactured not at all long apart, perhaps even in the same quarter.
In one corner of the board there are standoffs on which some setting resistors are placed. I'm guessing these are meant to set some limits for comparators in the analog domain. And sure enough, the -949 had different ones in there.

I made some photos of both ECUs, so if anyone is interested, let me know and I'll start another thread.

But the proof of the pudding is in the eating, so I went for a test drive.
I'm pretty sure the replacement of the ECU cured the problem. Not only does it drop back to 750 rpm idle all the time, acceleration when cold feels a little more fluid, less "wobbly" than on the wrong ECU. Other than that, the driving experience is pretty much the same as I was used to.

That transistor on the left in this borrowed picture (http://forums.turbobricks.com/showthread.php?t=227153&page=55) was the culprit of the non functioning injectors.
It is the driver for the injectors and was fried. A new one (the TIP-122) in its place was all that was needed to resurrect this ECU.

As for why the -561 doesn't work too well in my car, I'm not sure. It could be incompatibility or it could be that it's starting to fail. Who can tell?