It's pretty well know that in the past duck wasn't much eaten as a meat - a steady stream of eggs were far more valuable than the flesh. And traditionally the "standard" egg in most dishes was the duck egg - think of the Portuguese desserts of the Ayutthaya period, and things such as salted egg and phalo which, even to this day are still usually made with duck eggs.

I presume that in times gone bye chickens would similarly have been kept for their eggs, rather than their meat. (Though doubtless at the end of their productive lives they would have been stewed - let nothing go to waste.)

Similarly, it's known that some dishes now made with chicken were previously made with other proteins such as beef or fish balls.

So, (1) when did chicken eggs start to become more popular in Thailand? and (2) when did eating chicken meat become commonplace? (I speculate it may have something to do with the rise of the Charoen Pokphand Group (CP), but don't have any evidence.)

Yes, exactly. My understanding is that Dainin Chearavanant of the huge conglomerate Charoen Pokphand popularized chicken here. That would have been sometime in the last 30 years. Thais ate pork mostly before that.