Secondary Air DTC Inconsistency

Thank you. All true.
Replacing the combi valve made a huge difference, in the sense that I no longer got a fresh 0411 CEL soon after clearing the old one.
I did also replace the SAIP's outlet flex hose. The inlet flex hose should be irrelevant, unless blocked.
Having read that an old ("slow") pre-cat O2 sensor could prevent the SAI system monitor from clearing, I did replace it, as well.
At this point I am thinking either the combi valve is still not opening all the way or that the pump is not putting out as much air volume/pressure as it is supposed to. Everything in the SAI system was 22 years old, original with the car, except for the 3mm control hose for the combi valve, which I replaced with silicone a good 15 years ago, because the original one had cracked just past the combi valve nipple.
At this point I am down to the pump and/or the combi valve's control solenoid. Relay and fuse were the first things I ruled out, because the pump does run.

To me, the weird part is still why does the VCDS report "no faults" in generic OBDII mode, as my cheapo off-the-shelf scanner does, but "secondary air insufficient flow" in either autoscan or engine module reading modes? Why no CEL? Is the VCDS code somehow specious or otherwise misleading?

I did find an answer to the "pending DTC" question on nyvip.org:

"The PCM will successfully complete the secondary AIR system monitor if the proper voltage signal is detected from the upstream oxygen sensor. If an incorrect oxygen sensor signal is detected the PCM will set a pending code.[1] The PCM will command the check engine light to illuminate and store a DTC[2] if the fault occurs during 2 drive cycles." So, I conclude that my SAI self-tests have been intermittently passing and failing after I replaced the carboned-up combi valve. Before I replaced it, they had been more consistently failing. I'll see if my combi valve vacuum control hose fix completes the repair. I have ordered an F hose, per your suggestion, as a backup, since it is relatively cheap and will definitely need replacement at some point, anyway.

[1] detected by VCDS in scan mode, but not in OBDII mode
[2] detected by cheapo scanner and VCDS in OBDII mode

I'll give that a shot. Yes, the Bentley book shows a triangular plate with a mounting bolt on each corner.

I was a bit slow on figuring this out, but here is almost undoubtedly what happened, based on the discolored vacuum line with the split end and series of CELs and failure to set monitor:

The combi valve got carboned / clogged up over 22 years and did not fully close, leaking exhaust gas back into the control line to the AIR solenoid switch. I was eventually getting CELs because the combi valve would not fully open or close. Meanwhile, the exhaust backflow gradually carboned up the valve portion of the solenoid switch. Now, with the new combi valve, no more CELs, but also not enough vacuum to pull the combi valve open to set the SAI readiness monitor.

What I find interesting is that the SAI solenoid is driven by one pair (green/yellow and yellow/green) of wires from the ECU, whereas the secondary air pump relay coil is driven by a different set. I would have expected (and perhaps this is true) the pump and the solenoid to be energized together, but perhaps the intent is that the pump start before the valve opens and stop after the valve closes, to protect against the sort of backflow I was seeing with my defective combi valve.

Anyway, if I can't somehow clean out my control valve without damaging the plastic (I am going to try applying suction from its outlet to the combi valve), then my only recourse is to dive in and replace the solenoid. Thank you for all of your assistance -- this one has been a bit of a puzzler, and different from the usual leaky vacuum lines, bad pumps, and other common problems cited by others.

Not if the combi valve diaphragm has a leak in it. That is the only way I can explain the heavily discolored 3mm vacuum line with the hardened and split end at the solenoid control valve. I'll check the old combi valve more closely to prove or disprove this theory, which is the only one I can come up with that is consistent with the observations.

I have no vacuum suction from the control solenoid, and the direct combi valve suction test works, allowing blower air through the exhaust pipe, so if my theory is incorrect, I am out of aces at this point. With the old combi valve I was getting a fresh DTC within two trips, often just one, after clearing all DTCs. At this point, no more pending or full DTCs, but absolutely no luck setting the readiness monitor, and my VCDS reports -100% for secondary air flow when I try to force the unit 77 test, which aborts after two attempts.

F hose looks great. Other stuff I should be checking? Brand-new combi valve and brand-new Bosch O2 sensor.