For instance, exactly what is taken away from the trade routes?
a percentage of the trade routes output
How and when does this event occur in the game turn?
whenever a trade route is blocked at the time of its output (i.e. when the trade routes owner would have gotten the commerce/hammers/whatnot)
Which units have this ability to steal loot and how do they interact with other units?
I'd say they act exactly as privateers do
How does the pirating civ get the loot?
by blocking trade routes
Where does the loot go?
in the treasury or national pool
What should the requirements be before a unit gets to earn this promotion?
a specific tech, like the way you cant get mobility until youve researched exploration, if its to be allowed to all units, that is
What does this ability actually do?
it siphons gold from trade routes blocked
Does it take gold and resources from the city?
it would add negative value trade routes, that are not the full value, thus only taking a percentage
Does it reduce the trade routes profit, and how and by how much?
that would be variable in more complex cases, id say just 1 gold in simplest case
Percentage or finite amount?
percentage in complex cases, finite 1 gold in simple case
How do they come into the game?
ideally as UUs
Should players get to make them, or should they come from barbarians only?
definately player made, but no reason barbs couldn't have them. Lanun having them and Hippus, would be ideal
Are they invisible?
i wouldnt make them invisible, theyre more like privateers than spies
If so, which units can see them?
not applicable 
Do they have regular combat abilities?
regular with bonuses, like how pirate ships get gold for every ship they defeat
If so, they will also be able to capture enemy workers and settlers?
land bandits could capture the same way a normal land unit does
If you want to prevent this, how would you do it?
no need to prevent it
Does the AI know how to use them and to fight against them?
AI's won't like enemies in their lands, but its hard to say whether theyd know how to combat it; it would be better just to give negative diplomacy modifiers to deal with it