> I assume sending & receiving using her charter address will no longer be possible once our account there expires?
That's a question for Charter, but I wouldn't expect them to maintain her email account if there's no active (i.e. paying) customer account behind it.
As for the other questions...
> Will the years of charter emails downloaded to her devices still be on her devices?
That depends on more factors that we can know here. Chances are Charter are using IMAP which means, essentially, all the mail is stored on Charter's servers. They likely have some policy to delete old mails from inactive accounts, so it's going to go away at some point (unless you maintain a Charter account, even if you don't use their service).
However, for performance reasons, your Mac will download messages and keep them local (so that it doesn't have to keep in constant contact with the server). These messages will remain. However, you will need to maintain the Charter account in your Mail configuration (and Mail will periodically check in with Charter and bug you about not being able to log in).
However, this will still preclude your wife from either sending mail from her Charter account, or receiving any new mails.
> Will .icloud and gmail accounts work without specifying smtp etc settings?
iCloud and gmail.com accounts are configured automatically, and won't be affected by the switch from Charter to the new fiber-optic provider.
> There doesn’t seem to be a place to even input a new smtp server, etc. on the Mail app?
It is there, but it's convoluted. Basically follow
Mail -> Settings -> Account -> <choose account> -> Server Settings -> Outgoing Mail Account -> Edit SMTP Server List
> I assume sending & receiving using her charter address will no longer be possible once our account there expires?
That is correct. If your wife uses it for business purposes then you might want to reconsider dropping Charter until she has time to migrate business contacts to a different provider (or setup her own domain).
