A few updates upon further experimentation:
Upon restarting the editor, sampled lighting and lumen for local lights does appear to be working. I have no idea why it failed initially, as no settings were changed, but it does seem that RTXDI and lumen are in fact cross-compatible. This excites me.
performance is not good, holding at 30fps at 1080p in stable scenes, and as low as 26 fps with fast disocclusions. The test scene is the UE5 ‘open world’ level with a directional light and 130 non-overlaping point lights as a basic test scene. Upon restoring some errors in screen scaling, I’ve discovered that full-resolution RTXDI does in fact resolve reasonably well, although performance leaves something to be desired.
RTGI without DDGI volumes doesn’t appear to automatically utilize Nvidia’s NRD system, which is curious.