r.Lumen.ScreenProbeGather.ReferenceMode defaults to 0, so you can just omit it completely. Just don’t set it to 1 (non default) in the first place.
Once you are sure reference mode is off, are you really certain that turning off the importance sampling with r.Lumen.ScreenProbeGather.ImportanceSample makes the things better, not worse?
r.Lumen.ScreenProbeGather.ScreenSpaceBentNormal set at 0 is actually one of the things that destroys the contact shadows, so that could be part of the issue why your contact shadow is off.
But even if it wasn’t, for a realtime GI, that amount of accuracy is honestly already very impressive.
You seem to be just randomly turning cvars on and off and then using a “bro science” to arrive to some random combination of cvars, half of which have the opposite effect of what you are trying to achieve.
Anyway, for performance reasons, lumen doesn’t run at full screen resolution, so if you are interested in indirect shadow accuracy, your best bet is probably to find settings related to downsampling and fiddling with those:
I’d suggest trying to reduce r.Lumen.ScreenProbeGather.DownsampleFactor. It defaults to 16. You should be able to get more accurate indirect shadowing by lowering that value.