"This problem arose suddenly a few months ago, i.e., there were not periods of normal idle followed by intervals of high idle. "

This is a good clue indicating to me you will not need to go out and buy an LED test light, which is much better than a meter for catching something intermittent. Yes, a voltmeter will verify the TPS.

In a Bosch system, the TPS closed-throttle circuit will show about 5V when the throttle is opened, and near zero when closed. Most external functions in Regina are configured to match the Bosch, even pin-out, so I expect you should see the same. The idea is, to verify the switch is working, you see the voltage change.

A common misconception has the idle air control valve "closed" or "not operating" when the throttle is open, but the fact is, the valve is always under the control of the ECU, regardless of throttle, and it does not close. The difference is the feedback from RPM to regulate idle speed is only enabled with the throttle closed.

By providing some troubleshooting information, I'm hoping to save you from assuming you need to swap in a new VDO IACV, which may indeed be the cure, just in case the expense and trouble of finding one is not necessary.

"May I replace a VDO TPS with a Bosch unit? "

I have no idea. Someone with 7/9 experience will tell us, I'm sure.
--
Art Benstein near Baltimore

What Thomas Edison's Mother might have said to her son:
"Of course I'm proud that you invented the electric light bulb. Now turn it off and get to bed!"