Two Words: Chin Tuck.

Learn to move with the mechanical advantage your skeleton provides. When you sit, wherever it is (it could be either a round boulder or the most ergonomic chair you've ever seen), learn to balance your torso on your sit bones. That will also leave your legs free to move. Your head should balance on the weightbearing portion of your spine, leaving the spinal nerve free of compression. 

So, there is no problem if you want to lean, as long as you are aware of your skeleton position and let it support you, and your head sits/leads on the spine and leads its movement.

when i mewed without teeth touching i had similar posture but when i mewed with teeth together and i did a cheesy swallow well sitting down my posture fixed itself

The bad neck posture may depend on the chair? I thought your neck posture deteriorates if you don't sit properly. How can a chair affect your posture? I have sat in different chairs throughout my life, and I don't have any problems with my neck. Sometimes it happens that it starts to hurt, but I get up, rest a little and sit back down. I work sitting on a teak wood chair. Of course, if you sit all day, my back and neck will hurt, no matter what kind of chair you have. Therefore, I believe that the posture depends on how you sit.