The loop back within the admin is more than likely related to the 15 minute (typically that long but can be set to a smaller time) timeout that occurs while editing product info.

Two generally accepted methods to overcome this: install the keep alive timer or mark up your content outside of the admin and then copy and paste that content (preferably from a plain text editor) into the appropriate fields.

There is also Easy Populate v4 which would allow population of a large amount of information into the cart.

As to the sales/404 issue, in the transition you would want to have your previous uris redirected to the previously existing product that is now on your new store, though I say that with a heavy grain of concern.