Why do you think a prequest is needed at all?
It looks to me that you want to use the prequest to explain how slavery came to Whiterun. But considering that both the Imperials and the Stormcloaks have a strong dislike for slavery, introducing slavery would put Jarl Balgruf on the shitlist of both the Imperials and the Stormcloaks. He can't defend Whiterun against the Stormcloaks alone, and if he introduces slavery, he won't be able to accept the help of the Imperials. Basically, whoever wins the civil war would do away with him, trapping him in an unwinnable scenario. He's smarter than that. Thus, the prequest does nothing but add a very flimsy explanation to a highly lore unfriendly scenario. But what it does achieve is wasting the player's time.
Consider just not adding a prequest. That way you can allocate your resources, mainly your time and talent, to other areas where they would have more of an impact.
This part is fine.
It's a flyer, not a letter. He should say something along the line of "Please take this flyer from the slaverun company".
The player should decide how the PC thinks or feels about something. It's a RPG, after all.
Why is it called "the slaverun company"? The text indicates that the company already exists, so it wouldn't make sense for it to be named after Whiterun.
Secondly, Whiterun is neutral at the start of the civil war plotline. There's people openly supporting the Stormcloaks, and there's people openly supporting the Imperials. So what would not "not cooperate for war" even mean in that context?
This is mindless busywork and not fun.
People wouldn't tell that to a random stranger. Also, the prospect would probably be terrifying, after all, you could end up sold to a brothel or the mines.
Again, unfun busywork.
Would be shocking if the Jarl didn't know.
Why would the Jarl care what the PC thinks about it? Since this could potentially happen right at the start of the game, the PC is a nobody.
The idea of dominance and submission points is bad because it unecessarily limits the player's choices. Being opposed to slavery doesn't mean that you are submissive. It also imposes an interpretation on the player's actions. As I said earlier, I believe that only the player should determine what the PC is feeling or thinking.
Would be funny if you join the civil war on the Stormcloak side afterwards. Would make for an interesting conversation after he loses his throne due to your combat prowess.
Go to a place and talk to a person is not engaging gameplay. It's filler. And while you do need filler, only filler makes for a boring experience.
Please don't take writing pointers from Slaverun Reloaded, its writing is atrocious.