Its finally working!!!
To try to add the new Leviton ZW15R ZWave outlet, I initially used SmartStart (from the Hubitat app, devices page, + in top corner, Add Smart Start Devices... this was actually hard to find). Nothing happened at all. Outlet was already hooked up and powered on, maybe that was my mistake.
Eventually ran zwave exclude, which found and removed the outlet. At that point, I found the Leviton ZW15R in add device by brand, and ran its join network after putting the hub into ZWave inclusion, and it not only found the outlet, it used the name I'd given it during the failed Smart Start process?
Confirmed it showing as a device in ZWave, but no route yet, so I did a shutdown, power off for 30 sec, power up of the Hubitat, and now the outlet is not only working, but S2 authenticated.
Lastly, did a factory reset on the August lock. Reinitialized it, took it through the calibration process. Went into settings, Z-Wave Settings, joined to network. It caught fairly quickly, I gave it the 5-digit code from inside the battery cover, and it connected!
I did all of these things many many times before this, the only difference is the new ZW15R halfway between the Hubitat and the lock. Previously, the best route would have needed to use a lightswitch on the opposite side of the condo, or relay through a battery-powered thermostat... neither of those was a good option. I don't know if the relay being an S2 device helps, or shorter distance, or if it was simply a matter luck finally going my way. But, I'm hoping this stays solid!
(Lastly, to provide context, I intially had installed the August lock years ago, using their wifi hub. Battery life has been dismal, and I want the smarthome to not need an internet connection. So tried switching to zwave, and its been ugly... but hopefully all set now!)