Is there a downside when I force someone to marry me?

I would be very careful which NPC you select. Save before you do it.

Look them up in a wiki to see what quests they're involved in. Generally, NPCs that give quests will continue to give them after they move to a house, but there is a risk that the NPC might have a script or dialog condition that requires them to be in a certain cell or travel somewhere, and you could break that. I assume this is why you usually have to complete a quest for the NPC before you can marry them, as they're no long "needed" for quest scripting anymore.

Be very wary of NPCs that:

• Never leave a certain cell set
• Sleep for a short amount of time
• Are essential

As an example, Ulfric Stormcloak does all of these things to ensure he has high availably to the player, and he's involved in a lot of scripting.

In addition you should not do this to Khajiit/Bosmer NPCs, as they do not have the proper voice files and conversation topics to be spouses. (No Khajiit/Bosmer NPCs are available to be married.)

We might not be able to know for sure until we get the Creation Kit if an NPC is safe. By that time, you could just make you ideal spouse as a mod. In short, the console is really for debugging the game or mods you make and not for in-game editing of Bethesda's work.