Can't we all just get along and roll some dice?

If a player expressed to me that they were not comfortable with someone being brought into the game, I would simply respect their wishes and be done with it.

With only two players, I do feel like, if someone can't make it, you should just do something else. Run a duet one-shot, play a board game, whatever. Or get a third, unrelated friend to permanently join the campaign, if you must continue the campaign.

Underlying problem though: Susan's friends want to hang out with someone who's apparently wronged her badly enough she can't be around them. (I'm assuming you do want to hang out with Nora; if not, why is this situation happening at all?) This will not be solved by playing a board game!

I think an honest conversation about everyone's feelings is warranted here. Susan might decide that if y'all are going to do [exact same activity, but you've replaced her with her nemesis], she no longer wants to play DnD with you. I think there's a good chance the Susan-Veronica tension will devolve into open conflict eventually anyway. If I was Susan, I would've left this group long ago.

This is a messy situation to find yourself in the middle of. Just as a minor thing, you probably should have asked Susan before letting someone else have control of her character- even if it wasn't going to have any permanent consequences.

For your main question though, without knowing the details of the conflict between Susan and Nora, I don't think "Nora comes on the nights Susan can't" is a sustainable solution as things stand. Ideally, this is something that Susan and Veronica should work out between themselves, but if you as the DM and host need to make a call, I'd respect Susan's wishes and tell Veronica that Nora can't come.

I would never let someone play another person's active character unless it's been freely offered.
Playing RPGs is a vulnerable thing, even beyond just sharing a space with someone. There's a bit of social contract among players that we try to keep one another safe emotionally so that we can play this vulnerable game together without folks getting hurt.
If I were in your shoes, I'd ask Nora not to come back, and I'd apologize to Susan. If someone who had hurt me showed up at my game table and played my character, that would feel like a huge violation. It's kinda shitty to show up in the scene that someone we've hurt has invested their time and energy into. Sometimes it has to happen but when it does the decent thing is to give them what space is possible. Playing their character is the opposite of that. It's a major dick move. For a lot of us, characters feel like part of us. Susan's character used to be a part of herself that she was sharing with you. Now her character is part-Susan-part-Nora, in the memories of everyone else at the table.
I'd kick Nora from the game for good, and if Veronica can't hang with that I'd kick her too.

I stopped reading at you letting a random person play someone else’s character. Wtf drugs are you smoking? Mf krokadile?? 🤦‍♂️🤦‍♂️

You just don't want to bring in someone one of the starting players in the group has beef with. It won't be fun for anyone, and it's unfair to Susan. And you shouldn't have let Nora play Susan's PC, that’s like wearing someone else's underwear imho. Are you conflict avoidant? A people pleaser? Sounds like you have a hard time saying no to someone, tbh.

This isn't a D&D problem, this is a "do you respect Susan's wishes more than your friendship with Nora and Veronica" one.

We don't know what their "past beef" is. But it sounds like it was probably pretty bad and it sounds like, whether from a position of malice or not, you betrayed Susan's trust by inviting Nora.

I don't think, with the context here, you'll get good advice outside of the harsh reality that you need to make a choice between your friends.

"Sorry Veronica and Nora, this is a long-term campaign and I'm not planing on adding any new PCs to it at the moment. Your sister can join in our next campaign. The constantly rotating players is throwing me off"

Or something along those lines, would be my response. I had a similar situation with someone desperately trying to insert themselves into my current campaign for some "cameos", as they called it, and I literally said the above to them. I'm planning for my four players, I planned from the start for these four players. I don't need someone sitting in as what's essentially an NPC I have no control over.

Use this angle when kicking Nora from the table. This shifts any blame away from Susan.

I would find a different group to DM for, personally. Grown ass women acting like teenagers, smh.