Jeyne certainly won't be an Alicent figure. The biggest change for her is that she will be far removed from her mother. She goes with Robb precisely to escape her mother. Same for her brothers. They are all Robb's close friends. They fight and bleed for him.
So that's gonna bring them support too.
The Freys are numerous but also have horrible reputations and their attitude often leaves much to be desired.
Compare that to Jeyne's brothers. Knights. Decent behaviour. Loyal to Robb and most importantly- Seen.
Robb is seen with his battle guard. The heirs to many Northern and Riverland houses. They meet and become friends with Robb and the Westerlings. They fight and bleed together.
If you make Robb go on to rule the Vale and other regions, then as another anon said, those lords will see the Westerlings. They'll see Jeyne beside Robb. And even if they're disdainful because she's a mistress, the issue is that half the time Celia won't even be there to meet with them as Queen.
Robb doesn't want to take her because he wants the freedom to be around Jeyne without walking on egg shells. Also, he probably doesn't want her brothers around too much.
And after the Catelyn's betrayal, Robb doesn't want her near either.
So the problem Celia faces is literally that she's stuck in one place to build her "faction". That's gonna hinder her and also, she has a girl only.
And after Rhaenyra, Westeros just decided to be done with female rulers. Daena Targaryen got passed over for Viserys I her uncle despite Andal laws being for daughters before sons. So, Celia is already in a bad place.
Additionally, Robb will basically be a god to his men. The young king who never lost a battle and conquered half/full of Westeros and brought peace and justice to war torn lands. His gonna have incredible sway over men.
And he loved Jeyne. She's been beside him all his life basically if we go that far. At some point the people will realize that Jeyne isn't some fancy of youth. She ages, gives birth and changes over the years and Robb still loves her. She's not one of many whores like Robert had or ambitious like Alicent. She's just a girl in love. Who loves her kids and likely believes Robb when he says they'll be taken care of and that's enough for her.
I think it's more fun if Jeyne isn't like Alicent. She's not afraid of anything because she's always had Robb and in the end, it was all she wanted. Her babies were born with him by her side. He loved and raised em himself. They're so loved. What does she have to fear? Robb has conquered so much land, he can set them up for life even with a small land and castle somewhere.
Celia has that comfort atleast. Robb made her daughter his heir cause that's obvious. And he ensured the baby girl has everything needed to prepare for her role.
It's just that....Robb himself wasn't there. Because he didn't want to be with Celia. He kept her and by extension their baby girl away from him. He moved on in life without the ones who were there at the start. Catelyn, Celia, baby girl, Edmure, Sansa and Arya and later Bran and Rickon. These people were saved but then put away by Robb because he wanted more in life they essentially dragged him down.
You could make Robb ambitious instead. Celia just wants Winterfell. Jeyne just wants Robb.
Robb wants the world now. Because he's seen that he can take it. He's defeated larger and better armies before his 20s. He has the love and support of his Lords and men and he has a woman who loves and supports him unconditionally.
In a way, while orginally Robb just wanted to go home, here he just wants to move forward. Because home meant Celia and mother and his siblings who want him to be someone he can never be anymore. That boy died in the Westerlands.
He did his part. The North is free. His siblings are safe. He wedded and bedded Celia. He did everything expected of him.
Now he wants to be selfish. He wants to conqueror (I imagine Robb enjoys the conquering and winning battles part more than actual ruling) and be with his friends and be with Jeyne and Greywind and live. Live free.
He doesn't see how the new men (Vale, Reach and maybe more as they join) want him and his rule and to be a part of it. They want to make him theirs more than the North and Riverlands.
Robb both earns and loses parts of himself.
Jeyne and Celia just witness his rise and see the end coming. When he dies, their lives will change COMPLETELY!
Robb does. It fight for the Vale, but only for an independent North and Riverlands. Those are the two kingdom he claims and once that is accepted, he doesn’t try to claim other lands he has no right to. The Vale is his cousin’s, not his.
And while he would love to have Jeyne with him, he’s also aware of how dangerous and stupid it would be to keep a pregnant woman marching around. She spend a lot of time in Riverrun, although she campaigns with him a lot.
And just because Jeyne isn’t like Alicent doesn’t mean people won’t try to use her sons for their own gain. People dislike the Freys. It’s perhaps not as much as people dislike the Dornish when it came to the Blackfyres and their supporters, but still.
I don’t want Robb to be a conqueror. He wants independence for his kingdom and he wants justice for his family.
