Once dogs have been inside, they usually don't want to stay out and it sounds like she is much happier being a couch potato. She's obviously very smart and if you do manage to keep her from breaking into the doggie door, she could just break out of the backyard and end up getting hit by a car, stolen, picked up by animal control, etc. The best option for her and you is crating. Some obediance training would be a wise investment also. It's not cruel to leave her crated for 5-7 hours, she'll just sleep that whole time anyway. Leaving the TV or radio on will keep her company while you're away too (if she's inside).