The only way I've ever removed mounted artwork is to soak the mounted object in an Unseal bath (it usually takes longer than 5 minutes though, especially if the item is larger). Photos take longer since the papers are coated but newspaper will probably come up faster. Don't pull or force it (it might tear), just let it soak and do it's thing. Way back in the day when I worked at another shop, we unmounted a huge rice paper drawing successfully so it is possible as long as you take precautions.
This is how I do it. (and you could practice on a mounted newspaper scrap first)
1. Find a frame that's a little larger than the mounted artwork and lay flat on a table.
2. Lay an unopened plastic bag over the frame (I use one that hasn't been opened up and used so it's nice and flat and thin) and tuck it down into the frame, creating a plastic well (basically you're making a little lined swimming pool for the art to sit in). Be sure the bag is sufficiently larger than the frame so that when you pour the unseal in, it stays in the plastic.
3. Trim off any excess mounting board first and place mounted art inside the well (I leave it upside so I can see how well it's coming along).
4. Pour enough unseal over the artwork to submerge the artwork's surface (this can take half a can or more).
5. Let sit for however long it takes for it to unmount (could be as little as 5 minutes or as long as an hour). (Be sure to open your doors and windows so the fumes don't eat away your brain!)
6. When item is free from the board, let it dry completely. If you want, you can filter and reuse the unseal. I'd just label the can and use it for things like cleaning equipment.
