I received a new Idle Speed Air Valve and Throttle Valve Switch (TVS) today. I will probably just install the ISAV first and see how she responds to that and bypassing the currently faulty TVS' idle signal. Then I'll (delicately) tear apart the top side to gain access to the throttle assembly and replace the TVS.

Based on the helpful advice above, I'll order the injector insulating sleeves & o-rings. I've already got fresh rubber vacuum elbows for those connections at the throttle body.

2/14 UPDATE: I replaced the ISAV and started her up. No significant change to idle...unless I bypass the faulty TVS by jumpering its connection pins #1 & 2. Then the idle drops nicely...but still indicated on the tachometer as over 1000 rpm. Not great, but an improvement.

I went ahead and pulled off the Mixture Control Unit and Air Guide as an assembly to gain access to the throttle. As mentioned by another, it wasn't too hard (except for separating the Air Guide throat from the throttle).

The next step will be to see if I can reach the two TVS screws with the throttle still on the engine, or whether the throttle has to be removed first.

I'm semi-documenting this operation, in case others would like it outlined. See The 560SL Throttle Valve Switch Replacement Narrative.