I think you're on the right track.
You've tested that jumping back and forth on a call waiting call via a CC number works correctly using SIP.
I'd do a few more specific tests: Make two calls into your obi box calling your CC number (not by calling your GV number). Determine if you can jump back and forth when a call waiting call comes in. We expect this will fail in some way (on my Obi202 it cases either a hangup of both calls or a device reboot), not surprised it behavces differently on an Obi100.
If the fail proves true as expected, I'd suggest contacting CC and open a support ticket (noting that someone from CC states
here, link in first post, they work on all reported issues until closed, including the free accounts we use for the CC CNAM arrangement), rather than going through Obi support on this one.
Please post back and keep us updated, I'd like to find a resolution to the issue I was having before I decided to just disable Call Waiting.