He's awful young (5 months) so make sure you take that into consideration in all the training.
I agree that you may accidentally be teaching him that the way to earn a treat is to chew on something, mom says leave it, he looks at you and gets a treat.
Are you clicker training? Clickers are more concise cause you wouldn't have it get so muddy with exactly 'what' earns the click/treat. The 'leave it' gets the click/treat, not the chewing. AND when you are clicker training you have to go from click/treat 100% of the time to 'random' reinforcment and then to just praise most of the time.
You are a human and THINK you have trained your puppy not to chew the chair. It's perfectly clear to me as another human what you want. But your puppy is a puppy and it's not clear to them!
PUPPY CLASSES!!!!!!!!!!!!!!!!!!!!!!!!! Be a huge help for all the obedience questions like the crooked sit.
Though frankly, I would NOT train my pup to sit all the time when I stop on a walk. Way too much and too boring and no real reason. I WOULD however, instead teach my puppy to 'sit' at intersections! This is a SAFETY thing so our pup doesn't suddenly lunge into the road. And it's also a clear visual for the pup, for us to be consistant, and random enough so it's not boring boring boring.
Truthfully, the chewing the chair thing is just a normal bored puppy thing. With the 'even negative attention is attention' thing going on. I think if you can really up the exercise (off leash hiking for miles a few times a week going on yet) you'll see an immediate improvement in the random chewing.
PUPPY CLASSES for socialization and training at an appropriate speed.
EXERCISE for the both of you!
Heck, what I have to do with my puppies to have them wonderful in the house...