Yes, numerous studies have shown that workouts don't reverse the negative health consequences of sitting. That being said, such workouts can definitely help avoid (or at least reduce) other issues such as pain, e.g., by strengthening glutes that are weakened by sitting, etc.

It's not the sitting itself that is harmful, rather the host of tissue impingements and unfavourable biases in your musculature caused by sitting, or wearing shoes with elevated heels, etc.

Such issues cause us to compensate by adopting a different posture; one that will allow us to perform a desired movement now, but in the long run we suffer the consequences of this maladaptation.

An oft-cited example of a harmful adaptation is the proliferation of "heel-striking" running technique, which is a consequence of many people having poor ankle mobility (thanks, Nike).

To combat this, we can first establish what an ideal posture should be for a given movement, perform that movement under resistance, and use it to expose the areas in which we have a structural problem.

We then use that information to target problem areas (e.g. stiff hamstrings, shortened heel-cord, etc) with myofascial release and stretching.

Make mobility work a part of every workout you do (15-20 mins a day): use foam rollers/lacrosses balls to remove knots; use "banded distraction" to open up stiff joints. In my experience and anecdotally, such effort yields excellent results for pain-relief and improved posture.

I highly recommend Kelly Starlett's book "How to Become a Supple Leopard" and Steve Maxwell's mobility programs.

Well it's not great. You should really get up and move around during that 8 hours.

I attribute this to most of my lifting injuries. Unscientifically it strikes me as patently unhealthy to go from one extreme to the other without a long warm up period.