I tried going guitar:input 2 > bus 34 > bus 35 > Helix Native side-chain input > stereo out. Initially it worked with no latency and turning on low latency mode gave no sound at all. But after Logic Pro re-opening the project, the latency is back and turning on low latency mode gives the dry sound routed to the stereo output, even through there is no path that I have configured that would do that. i.e., I deleted the aux track for bus 35 > Stereo Output.
I was able to get no latency to work in a few other instances, hard to know exactly how. But again as soon as the project is closed and reopened, the latency is back.
I'm moving to Reaper for this application. Its routing, especially for parent/child tracks and MIDI configuration is just much simpler and works perfectly.
I love Logic's UI, workflow, instruments and plugins. But without side-chains into MIDI controlled plugins working, it's not useful for guitar plugins, which is a blocker for me. I'll file a bug with Apple, but don't have much hope it will be addressed anytime soon.