What will be the full price once released?

The full price for Mono will be $69.

Will it be possible to copy from a memory card reader to two external USB HDDs simultaneously, with a USB-C hub?

Mono is for now limited to a single source and single destination, as the iPad doesn't really pack the punch to do more. Copying a card to two external drives over a single USB bus is a recipe for disaster, so it's best to offload first to the internal SSD.

If I copy internally an SD card to the iPad's internal storage, can I then copy that to an external HDD afterward?

Yes.

If I copy for example RAW photos from an SD, could these be previewed and imported in the iPad camera roll?

For that, you'd need to add them to your Camera Roll first, and then make sure the iPad supports the format you are using. Don't bet on a lot of codec support from Apple on iPadOS.

Are the copied files available within the Files app? Or Photos? Or only seen and managed from within Mono itself?

All within Files, and none within Mono. We still don't do vendor lock-ins 😁

Can Mono handle folders too? Or only drives as a source?

Folders too, and even files - like with Hedge.

Will duplicate detection be available?

Yes, although not as deep as Hedge's dupe detection as the iPad doesn't reliably allow for modification date comparisons. Thus, dupes are checked for filename, size, and of course path.