A few questions:
What is the difference between a nendoroid and a petanko?Nendoroids are made by Good Smile, They are chibi versions of their scaled counter parts. Pentako are made by Penguin Parade. They are seated chibi's because the word petanko can mean; Flat-chested/ seated girl.
What does Figma mean?Figma is what Max factory calls thier action figure line. Figma's are basically figures that can be posed.
How do I know if a figure is painted or unpainted at RightStuf?Don't worry, most will tell you flat out if the figure is painted or not.
How can I tell if I'm slowly becoming addicted to anything chibi?When you actually rather have the chibi over the scaled counter part... you just may have a problem.
Are nendoroids sturdy enough to handle mild use? Say... people coming over to your house and saying: "Oh waht's this? Cool." and picking it up and playing with it. Or are they something you protect behind glass? Furthermore, does repeated changing of accessories wear out/loosen the fittings?
Finally this one.
1. Yes and no.
Yes, they're very sturdy little figures.
No, cause depending on what kind of base they have, they just might break the Nendo off of the base.
I wouldn't let people play with them as; answering the other question of yours, the joints do loosen with use. However, a little superglue applied around the lose joint does fix the problem.
Hope that was helpful for what it's worth.