The initial brake pedal action is full regeneration (regardless of whether you are in B or D mode). I believe B alone provides up to 0.12 g of deceleration and the initial brake action increases that to 0.25 g before the mechanical brake are blended in if Kyle Conner's notes were correct in one of his ID.4 Out of Spec YouTubes. A 15% grade requires 0.148 g to maintain velocity, so yes, B alone is not enough but when you press the brake pedal you are still well within the full regen region.
In reality there are three modes, D in comfort (no regen), D in sport (mild regen), and B (stronger regen). I think you need a very steep grade to exceed the regeneration of B mode. Natural regeneration diminishes as speed gets closer to zero. To have very strong regen down to zero you need to run the motor in reverse.
I recently went up to Crater Lake, Oregon. Left Klamath Falls with 200 miles. Got to Crater Lake with 65 miles left (Klamath Falls to Crater distance is 70miles uphill). Left Crater Lake to Eugene which is 140 miles away. I drove the entire way using B mode, got to Eugene with 70 miles remaining. So the B mode regeneration was quite impressive. This was part of a bigger road trip I made from San Francisco through Oregon and Washington and back home for a distance off 2200 miles using strictly Electrify America. So I paid zero dollars to charge.
