search or look for files most recently or newly hashed or indexed in filelist(s) or arrange by date

Once it's finished hashing it will appear on your share on the next refresh. Use the search engine & search the item for it's Tiger Tree Hash or TTH.

On the search engine, the item will be displayed by File, User, Type, Size, Path, Slots, Connection, Hub, Exact size, IP, TTH & CID. I don't think it can be found by date modification.

You can check the difference between a current and a previous state of someone's filelist by using the 'Substract list' function at the filelist's statusbar. Of course you need to have an earlier state of the filelist saved. Yo do this you need to make a backup of the user's filelist after you've downloaded it. Next time you get the filelist then use the Substract button where you can specify what saved filelist you want to compare the current one to.

thanks, it would be nice if DC++ had the backup process as a feature (e.g. always retain old lists option).

Relatedly, is it possible to list all files in a list, for instance that subtracted list, or any filelist in general, without respect to folder structure (the output would be like if *.* was searched for within some directory, the output would list all items in a list regardless of subfolder position, and sortable on folder position, or any of the usual attributes, such as size, etc.)

You can retain filelists by enable the according option in Settings/Advanced.
The other thing is not possible atm, and I doubt it'll ever be, but you always free to file Wishlist requests in the bug tracker...

that thing doesn't sound like it is too hard to do, are there ways to make plugins?

No, there's no plugin interface. As I said you'd better to file a bug report about your idea so all the developers can take a look at it.

https://bugs.launchpad.net/bugs/371053