Since not all models are the same as they age, what should I pay extra attention to when inspecting/riding a used bike for sale that may have greater than 10k miles?
You've somewhat answered your own question. The condition of the bike is more important than the mileage. Inspect everything.
I assume inspection of tires, battery, brakes, electrical functions, cable controls and clutch/trans wear, but what else ?
Check the chain and sprockets if you'd like, but those are fairly cheap and easy to replace. Use their condition more as an indicator in how well the bike has been maintained. If they're completely trashed, that could be a sign the rest of the bike has been neglected.
When a TW200 goes down what is commonly damaged or bent?
Handlebars, levers, turn signals.
Are cold starts on a older bike more likely to give a blue puff of smoke than newer bikes?
Newer/older and high mileage/low mileage don't necessarily correlate to the condition of the engine. If the bike puffs blue smoke when starting, regardless to age or mileage, be prepared for an engine rebuild in the near future.
How smooth should the gears shift if in good condition?
What about clutch engagement?
As smooth as Keith Stone.
How long do the drive chains usually last on this bike if used normally and not abused?
I don't know on the TW, as I've only ridden mine 1000 miles so far, but based on experience with other bikes, I'd say 5000 is around average for an o-ring chain, maybe 1000 for a non-oring, again depending more on how the rider took care of the bike and what conditions the bike encountered (sand, mud, salt) than mileage or age.