I have egtw and my friends want to run a mighty nein campaign. I’m trying to get my head around to memorizing most of the cities but if you guys have any tips I’d love them.

They don’t want it to follow similar plots, but love the campaign and thanks to most fandom stuff i have a lot of stores and other info. 

Any tips that would help on making fun adventures? I don’t want every single quest to involve the myriad or other organization. Any help would be appreciated

And if you have any ideas for running arms of the betrayers, I’d also love help with those

You don't need to memorise every city or town, thats just silly to try and leads to being confused or muddled when you describe things. Instead, make sure that you know the details about the starting city, and maybe the notable traits of the nearest towns/city e.g theres no roads, just canals. This means you don't become overloaded but can still give your players the necessary info about the place they're actually in. 

Also don't try to recreate the mighty nein campaign. That just turns it into a railroad and doesn't allow the players agency in their own story.

My advice would be to not try and emulate the Critical role campaign: its made up of professional voice actors and they all have the time and experience to develop complex stories with one another, not to mention that its also a show, so they kinda need it to be entertaining. 

There's nothing wrong with making an adventure in tbe setting and making it very focused on roleplay, but becoming so focused on the Mighty Nein as the standard will leave you disappointed if you're game doesn't play out the same.