If -and only if- you have enough patience, it is not hard to remove the backside. Keep on heating the backglass, but not to warm. Around 80 degrees celcius for 5 minutes. Pry with a plastic tool around the edge and keep heating it up if it takes too many effort to come loose.
You can't transfer the data if your battery is dead. Your only solution to get your data back is to replace your battery. When you succesfully removed the backglass, replacing the battery is easy. Just be careful to not pry the battery, as it can get on fire.