I solved my problem with this by adding | WRITE_BIT to each of the write_byte calls. Not sure what it does exactly, I'm still quite new to C and I2C too :) I'll dig further to work out exactly what's happening, I'm not the type to just settle for 'it works' :)

void writeI2c(uint8_t regaddr, int ...

I'm having similar issues, it's certainly a more complicated library than the Arduino 'Wire'.
The examples are actually difficult to follow, very complex scenarios and instead we'd benefit from a simple master write / slave read example.

Did you add any value to the timeout method call?

I've ...