What does 'on so' mean and how it changes the meaning of the phrase?
Yes, I was going to ask Mr. Quinn why he insists on putting us and the rest of the world on so?
This is tricky because it combines a two-word verb (verb plus preposition) that is a separable two-word verb with an idiomatic usage of the word so. Separable two-word verbs can take direct objects and here, the direct object is a compound direct object: there are two of them.
The verb is: to put on, and it can take direct objects in typical usage such as:
Don't put me on.
Don't put us on.
You're putting us on, aren't you?
So is used to mean: so much or to a great extent and it placed after a verb in certain cases:
They do go on so. = They do go on so much. go on=to make a fuss, to whine about something.
She always is complaining so [so much] about her job.
He never used to whine so. [so much or to that extent]
The parse is therefore:
he insists on|| putting us and the rest of the world on|| so
The difficulty, therefore, is three-fold. There is to insist on (correct preposition for the verb insist), to put [someone] on with a double direct object (us and the rest of the world) and the so standing for so much.
who said English structure was easy?:)
Parse this as putting on with so="in this manner" or "to this degree" as a modifier.
Put on here is extended from the metaphor of "donning" a disguise or false identity to mean "deceive" or "trick". You're putting me on means "You're kidding me, you're trying to trick me."
I was going to ask Mr. Quinn why he insists on deceiving us this way.
Note that the following line is
Some might be persuaded to doubt his sincerity