Is it possible to copy cities such as Solitude or Whiterun and put them in their own cell? Basically so I can create an alternate version of the cities when I need to

They are in their own cells, or rather "Worldspaces" - unfortunately, no one apparently knows how to actually make worldspaces work without using multiple additional pieces of software besides the CK...presumably you could should be able to copy entire worldspaces to make "Alternate Solitude 01" or whatever.

The CK doesn't have an internal function to copy an entire worldspace.

You could do a variation on what I do with Open Cities though. Make your new worldspace. Set Tamriel as a parent to it. That solves your landscape and LOD issues.

You'd then have to go cell by cell within SolitudeWorld and copy each cell from there and paste it into your copy. Which is a hell of alot of work and will require you to make numerous alterations to AI, scenes, quests, etc to account for using the duplicate when the time comes. You'd have to take care not to duplicate NPC refs while you're at it.

In an ideal world, all is well once you've done that. The problem is, Skyrim is in a less than ideal condition. The navmeshes alone would make such a project impractical right now. You'd end up with all of the same issues I've been facing since February. Perhaps more since you'd also have to edit AI and stuff.

Solitude and Whiterun also have huge civil war conditions to keep in mind. That alone is a massive pain.

I suppose if you wanted to make a "100 years ago" version of a city, copying only the statics and making all your own NPCs, etc...

Word > Worldspaces > Duplicate seems to work. Just made a copy of JaphetsFollyWorld.