Minecraft – How to change an entity’s item value

This answer was written prior to the release of 1.13 and "The Flattening" that came with it. An updated answer can be found here.

You're actually pretty close, and definitely on the right track. Instead of {Inventory:[{id:<block ID>,data:<data value>}]}, use {Inventory:[{id:<block ID>,Damage:<data value>s}]}. The s after the data value is important since it's stored as a short, not as an int. (As an aside, if it were stored as a byte, you would append a b instead.)

Based on what I've found by looking at commands, I think you want to do

/summon Bat ~ ~ ~ {Passengers[{id:"Villager"}]} 

That will summon a farmer riding on a bat. If you want to do things like giving the villager trades or changing his profession, mcstacker.bimbimma.com will probably help.