I talked to Maramal, bought and equipped the amulet of Mara, went to Riverwood, and talked to Hadvar. However, the marriage dialogue did not appear. Just to make sure I entered the command correctly, I talked to a merchant NPC and it worked fine. Does the command not work for characters involved in a questline, such as Hadvar? Will the command work and will my character be able to marry Hadvar once the Civil War questline is completed?

Some NPCs just aren't the marrying kind.
They don't even go for it when you try to console them into it, probably their game logic package stopping it from happening.

certain prescript-ed NPC just cant be console overridden

Why would you want to console an NPC into marrying you? You don't get all the benefits or conversation that comes with the marriage mechanic that way.

Marriage is based on voice actors, not characters. Thus you can't marry most NPCs with unique voices like Hadvar, Tullius, Elenwen and Ulfric.

It may be a case of the dialog flat out not existing, just like for the Khajiit. AFAIK Hadvar's voice is unique in the game; if he's not intended to be married, then the lines for it wouldn't have been recorded.