Programming an MCU with an unknown toolstack/SDK
The SG8F080P is a processor with OTP - One Time Programmable memory. So you can't reprogram the existing circuit.

More, even if you could re-program the memory, and want to integrate something else, you would need the entire source code for the original functionality, and it is very very unlikely that you can get it, ever. Their proprietary code is likely their biggest asset, likely contains some know-how to handle the touch sensors reliably, so they won't give it to you. So your whole starting premise is wrong.

The CAP1188 is not the MCU, it is a peripheral device. So your best option is start with something known, public, Raspberry-Pi Zero, Arduino, Particle Photon, and add the sensor chip, and the radio (if it is not already there). Basically, you need to start from scratch.