I'm a Lightroom plugin developer, one of my products is The Fader (https://www.capturemonkey.com/thefader/), which can crossfade between different develop settings. It operates on normal sliders, but it can also create smooth transitions between tone curves.
In order to operate, it needs support from Lightroom to set sliders and tone curves. The SDK provides few methods for this:
2) LrDevelopController.setValue, which has been my solution since Lightroom 6.0. It has a few quirks, but it did get the job done. ToneCurvePV2012 (/Red/Green/Blue) were not mentioned specifically on the documentation, but they were working in LR 6.0-6.8, something along the lines of: