Extremely interesting technical discussion here related to your idle problem -our experts are seemingly fully engaged for you. Thanks for posting those links with the LH 2.4 Jetronic Volvo Service Manual and Pinout/Connector Diagrams -pure gold.

As for your thoughts on swapping out your -561 ECU, here's a few compatibility tips that may help with sourcing.

I note from Bill Garland's ECU page (showing primarily North American market applications) that the LH 2.4 -561 ECU was used on the B230F in the 240 (1989-1992?, with 1992 more likely to be LH 3.1) and 740 (1989-1990). The original pink label ones were known to be problematic (most notably the fuel pump control circuit could fry, a no start condition, also affected the pink label -562 used in the B234F). The revised white label one that you have is known to be more reliable, but my recollection of user reports is that it's still not as reliable as other variants.

The -933, -935 and -951 are listed as good substitutes, but I'm wondering if you need to be careful about EGR vs. non-EGR (yours). Bill's tables suggest it may not be an issue for you and that makes sense as EGR is mostly on the EZK side. If there is an EGR pin on the ECU (I haven't looked) then to make an EGR ECU work without EGR you might need to so something like ground or sever the cable pin.

The difference between variants can be as little as the PROM, so opening them up and doing a PROM swap is an option to consider if you get stuck sourcing a suitable replacement.

On your raised idle problem in general, my first thought and I'm sure that of many was the ECT, but your diagnosis suggests that's out of the equation. Another thought is always the A/C being engaged, but you say you don't have that. Rather grasping at straws here and your diagnosis appears to be beyond this, but I'll mention them anyway as these are common idle problem areas. While idling too high or too low, don't forget to give the IAC a few good taps to make sure it's not sticky. I did like thoughts about the throttle plate and the TPS not being adjusted correctly, so do double check that the TB throat is clean, check the stop screw adjustment (using the 1/4 turn method) and make sure the TPS isn't set on the hairy edge of triggering, plus make sure the TPS is in good working order, with your DVM seeing a fairly crisp and solid cutover to indicate the contacts are in good shape.

Oh, and to most of us here in Canada and also Australia/NZ, probably SA as well, it is colour me grey, not color me gray, so that was not overly revealing of your locale or heritage, but the 2.0 litre thing does rather suggest your continent. Be on the lookout for one of those rare 16-valve 2.0 litre 700s (possibly an early 900?) that found their way into the Italian and Belgian markets in the late '80's (and early '90s?). Reportedly, the B204F (forerunner of the B234F) produced something like 170 HP, while the B204FT produced something like 190 HP, pretty impressive by brick standards (there was no B234FT). I thought that was an ingenious way for Volvo to deal with the 2.0 litre taxation issue using existing engineering (in conjunction with SAAB?) and production capability.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now