[GeoffR]

You only need ADE if you want to sideload books with Adobe DRM. If you only intend to sideload DRM-free books then you don't need ADE at all.

You can sideload DRM-free books by copying them to the device or by sending from Calibre via USB, or you can download them via wi-fi from a Calibre content server, dropbox account, etc. (There are some advantages to using Calibre to sideload the books via USB, as it can manipulate collections, send better cover images, etc.)

Registering the device doesn't require a credit card or any other personal information other than an email address, which can be a throw-away address if you want.

Registering the device doesn't involve ADE at all. You don't need to download ADE or create an Adobe account to register the device.

If you do choose to sideload DRM books using ADE then you will need to authorise the device with your Adobe ID, but that is a completely separate option to registering the device.

Edit: There is no need to install the Kobo desktop software on your PC either, unless you don't have a wi-fi connection, as registration/setup can be done via wi-fi from the device itself.