Do you find that timing chains last longer than drive chains? Yes, by quite a bit. Again, all the sync chain does is transmit the front rider's power to the rear crank axle at a 1:1 ratio. The drive chain must handle the combined load of both riders and must deal with the added wear and tear that comes from the very small cogs as well as the higher torque of the 'granny' gears if a team rides in places where the alpine/granny/small chain ring actually gets used.

Does it make sense to rotate the timing rings each time you replace the chain in order to spead the wear? You can and it does work; however, it can become more trouble than it's worth relative to keep track of how long a given chain's been in use. I gave up on this practice a while back for both the sync and drive chains and use my limited attention span to check wear a bit more frequently.

CC-2 tool measures stretch as a percentage of chain length; Why is that? It probably seemed more intuitive to use a percentage for consumers vs. 1/32", 1/16", 1/8".

do chains have an initial stretch or "break-in" and then stretch slowly after that? That's been my experience with both motorcycle and bicycle chains. There are a lot of variables here, right down to the different brands of chains, the types of lubricants that they are shipped with and how temperature might affect their viscosity, not to mention how someone might prep their chains for use. Smart folks who pile on the miles seem to slap on a chain with the factory lubes and slather on additional lubes only if the chain begins to squeak before it's time to chuck it. Me, I strip off the factory lubes and do my own thing once the chain is clean.