Thanks for sticking with this. I had a look at this thread again and the GRID configuration and found the solution:
* In GRID Autosport change the controller to Handheld, Pro Controller, Hori wheel and ensure current config is "Classic Analogue" - see attached. This maps the accelerator to right stick up and the break pedal to right stick down
* In the S1 app simply map the right stick up to 'TP' on the G29 and right stick down to 'BP' and you're done
Now you have analogue breaks and throttle control

Note: Based on the "classic analogue" profile ZL and ZR are mapped to gear down and up respectively but I haven't tried this yet.
So you may want to update your GRID Autosport profile for the app (breaks, throttle, gear up/down, game dead zone to 29, FFB to 15) and give the G29 users out there a lot more fun

Let me know once you have updated the profile and I'll test it