I think mashing down the accelerator will override phantom braking. Definitely, one can disengage FSD/Autopilot/TACC and take over.
How quickly a driver can notice the problem, decide that the car is not braking for an actual obstacle, and get it back up to speed depends on how alert and prepared they are.
These are ADAS "level 2" ("class 2" might be a better term) systems The 6 Levels of Vehicle Autonomy Explained | Synopsys Automotive
Yes, pressing the accelerator will override all braking, phantom or otherwise - you don’t even need to mash it. The only time this isn’t the case is if the car detects an imminent collision and engages emergency braking.