not to be put on and off

[keannu]

You can only answer one of these, please don't be burdened.

1. Does this "not to be put on and off" mean an obligation "should not be put on and off" as one of the "be to infinitive" usage or just "something that is put on and off..."? The translation is the latter, but I lean toward the former as the latter doesn't belong to any one of "be to infinitive" usages.
2. "sit on" is translated as "fit", but I can't find such definition in the dictionary.
3. Does this "observances" mean "obeying or following"?

330-32
ex)Good manners are not to be put on and off with one's best clothes. Politeness is an article for everyday wear. If you wear it only on special and rare occasions, it will be sure to sit awkwardly upon you. If you are not well behaved in your own family circle, you will hardly be truly so anywhere, however strictly you may conform to the observances of good breeding, when in society...

[Barb_D]

This whole thing reads like someting from a primer on manners from the late 1800s. Is this really something that you need to fret and worry over?
Here's a modern version:
You shouldn't think about good manners the way you do your finest clothes: something to be used only on formal occassoins. You should be polite all the time, and if you do reserve good manner only for special times, you might find it feels awkward to use them. Even if you consider yourself well-bred, if you aren't polite at home, you probably won't be polite when out and about.

[birdeen's call]

2. "sit on" is translated as "fit", but I can't find such definition in the dictionary.

"Sit" can mean occupy a place. Politeness cannot occupy a place because it's not a physical object, but clothes can. And politeness is likened to clothes in this paragraph.

[keannu]

I'm not desperate to analyze it, but I have the responsibility to explain this to others as it's a question of a mandatorily designated workbook for university entrance exam. Anyway I'm so amazed at your skill to translate it into a modern version so easy to understand. Awesome!!! Thanks a lot!!!