You sign in with your Google account. The app downloads the selected emails to your computer, removes attachments from the local copies, inserts these copies into your inbox, and then removes the original emails from your inbox.
The tutorial video shows the steps in practice.
Yes, you can easily use it with multiple Google accounts. Simply sign out of the current account (Account menu > Sign out), and sign back in using a different account.
Note that settings are saved per every operating system (OS) account, in
.unattach.properties of your OS account's home directory.
No. Unattach versions use the semver schema: <major>.<minor>.<patch>. Minor and patch upgrades are free, while major upgrades are either free or offered at a large discount.
Yes. While there is no first-class support for these, you can install their local sync-ing app, and then configure Unattach to save attachments within a local directory that's sync-ed with the cloud.
Note that doing so with Google Drive won't free your Google storage, since it is shared across all Google apps (Gmail, Photos, Drive, etc).
Unattach is operating on a relatively low budget, and cannot afford an expensive marketing campaign. The app's ranking is slowly increasing on Google search results.