An idea of how multiple tool instances for multiple local adjustment points could work:
Every tool gets a new "instance" combobox at the top of the tool frame/vbox. Instance 0 is always the global effect the tool has, as every tool in RT currently works. But you could add a new instance of a tool which could be used by the Local Adjustment tool to adjust a small area, or it could be also global, working as a layer.
For example you set exposure compensation to +1EV globally, instance 0.
When you add a new adjustment point you select the tool you want it to use, e.g. Exposure to brighten up the left eye in a portrait, and place the local adjustment point on the preview. A new instance, 1, is automatically added to the Exposure tool's instance combobox. The Exposure tool's sliders are reset for instance 1. You set exposure compensation to 0.5EV for instance 1.
You can add a second local adjustment point also for the Exposure tool, so it gets a new instance, 2, for the right eye. +0.7EV.
You can select any instance from the combobox at any moment - this sets the sliders of that tool to show the settings which that instance uses. If you select instance 0 then the Exposure Compensation slider moves to the +1EV position, if you select instance 1 it moves to +0.5EV, if you select instance 2 it moves to +0.7EV.
The main preview shows a dot with a number, this number corresponds to the instance, so you can see "Aha, the left eye uses the exposure local adjustment instance 1"
PP3 handling: each tool section gets an instance index, so
[Exposure] changes to
[Exposure#2], etc. If an instance of e.g. Exposure is used locally, then the Local Adjustment tool gets a
[Local Adjustment#1] section.
The local adjustment tool would not re-implement each supported tool as it does now. Instead, the tools would be made to handle multiple instances. Perhaps the output of instance 0 would go into the input of instance 1, if it exists, or into the input of the next tool's instance 0 if the current tool does not have multiple instances. So the pipeline would still be fixed but allow multiple instances of a single tool.