-What happens if you try reading the MPR121 but the MPR121 is in the middle of gathering it's sensor (electrode) data?
-What happens if you try reading the MPR121 and in the middle of that the MPR121 needs to sample its electrodes at the same time?
guessing: Nothing. Even assuming that the MPR12 contains a microprocessor that does the sorts of things you're asking about sequentially, the pause at the level of external I2C behavior would be imperceptible, since I2C will usually have hardware shift registers and some buffering, and I2C is quite slow compared to modern microprocessor speeds.
Can you suggest a keyword that I should search for to find this information?
Something like "WARNING"? As in "warning: if an I2C command arrives during a sensor scan, response can be delayed up to nn clock cycles." Or perhaps "warning: the MPR121 processor should only be written to using I2C when it is not busy scanning its sensors, as indicated by the "IMNOTREADY" status bit."
You can get a more visceral feel for the possibilities by imagining code written for the AVR that would be an I2C slave, reporting data from sensors connected to IT.