The ACES shaders are in Engine/Shaders/Private/ACES. And the result you get is dependent on CVARs that begin with r.HDR.
The ACES code contains the HDR tonemapping algorithm, so I’ll admit I didn’t quite understand question 2&3, because the HDR path is also ACES.
Can you share the various r.HDR CVAR values that your project is currently using?