I think we’re getting pretty fair afield of Heinlein’s idiosyncratic usage of the word here. Aren’t you both describing the standard, common conjunctional use of the word “so” that we all use? “Our options are going out to eat, ordering a pizza, or eating leftovers. So, which one should we do?”
@CalMeacham brought up specifically Heinlein’s tendency to have a character begin a line of dialogue with “so?” Note the question mark.
I still have on my bookshelf copies of three Heinlein novels. I just leafed through each book to find an example of this, and it didn’t take long.
From The Cat Who Walks Through Walls:
"Tony, could you please have some female member of your staff check the ladies’ lounge for Mistress Novak? I think that it is possible that she may have become ill, or be in some difficulty.
“Your guest, Dr. Ames?”
“Yes.”
“But she left twenty minutes ago. I ushered her out myself.”
“So? I must have misunderstood her. Thank you, and good night.”
From The Moon is a Harsh Mistress:
“Manuel, you may be. the oddest man I’ve ever met.” She took that print-out. “Say, is this computer paper?”
“Yes. Met a computer with a sense of humor.”
"So? Well, it was bound to come some day. Everything else has been mechanized. "
From Stranger in a Strange Land:
She stood up. “Is Doctor Nelson likely to come popping in?”
“Not likely, unless I send for him. He’s still sleeping off low-gee fatigue.”
“So? Then what’s the idea of being so duty struck?”
I also noticed plenty of instances of characters using the word in a more typical way, saying things like “so, what should we do?” Or asking “how so?” Or “But we don’t have any red paint.” “So paint it blue!”
Therefore I think the “tic,” as it were, is specifically the use of “so?” as a one-word question where, as the examples above demonstrate, it doesn’t really make sense to replace it with “so what?” or “therefore” but rather “is that so?” or “really?” And that is certainly unusual, because I’ve never heard anyone IRL use it that way, nor encountered it in any other author’s dialogue.