I see that she's essential so I can't kill her but is there any other way to hurt her? Are there any quests where I can actively work against her?

I'm RP'ing as an Imperial agent so she'll end up as the Jarl but I still would like to stick it to her somehow.

Not really that I know of. In the Theives guild quest you can burn down all the hives, which is suppose to make her have to import the honey for the mead.

I one day hope I can either kill the Black Briars or utterly ruin them to the point of povety

Hey now, Ingun Black-Briar is ok.

Completing "Promises to Keep" without ratting Louis out is one way to stick it to Maven.

I killed Ingun in a rage filled Werewolf attack, and almost died as I ran away after finding out both Maven and her son are immortal

I killed her son in prison. He was so happy when I lied to him that Lynly fled to Morrowind....then he took a Daedric Arrow to the jugular. I only paid 40 gold for my bounty anyway.

So I thought, too. I completed her quest. Much later, she mentioned that she's only interested in the dark side of alchemy, so she's carrying on the family tradition in her own way.

Just side with the Stormcoaks. This is the ultimate revenge by stoping that arrogant woman from ever becoming Jarl. I did the Imperial side first, and wondered who & why in the hell did I help this woman become Jarl of the Rift?

Whatever. Maven will eventually die and her much nicer and acceptable daughter Ingun will take over.

Sez you.

How do you know she doesn't plan on poisoning Mommy Dearest herself... if only the PC could accomplish such a feat.

EDIT: If you complete her quest, every few game days, you can loot her chest down in the alchemy shop for whatever it's worth... at level 36+, it's worth a LOT. Blacksmith's Elixirs, Enchanter's... Etc...

If you're not an Alchemist yourself, it's a never-ending treasure trove.

And, no, she never back-stabs you.