> Is the sync feature only available via Pocket or could you also use Dropbox?
The sync feature is only available via Pocket
> If I want to save a page to archeeve from my browser, would I have to do it via the Pocket browser extension?
Yes, you can use Pocket browser extension.
> When I clicked on the screenshots, it took me a moment to find how to close the modal. The close link is kinda hard to see because it's not near the image and it's quite dark (it almost seems greyed out).
There is no the close button/link (I think you mentioned about the remove button) :) You can use 'escape' key on your keyboard or click on the bookmark once again.
Currently the app is available for OSX, however in the next month it will be availble on Windows too. The app is created with node-webkit. Let me know if you would like to discuss about a technology I use in the project.