Questions regarding meditation posture and movement I was trying sitting cross legged and back straight during Anapanasati meditation.

I have looked at this question -> A question about sitting, movement and mindfulness

I would like to know what are the possible bad practices learned during meditation and in the posture and I would not want to make those mistakes.

I am not sitting purely cross legged. The feet are under the shins or calf. I am unable to sit perfectly straight noticing that I am leaning forward. Should I at this juncture, straighten up again? What if a mosquito bites you at that moment, like it is biting me right now? Should I move to scratch an itch?

Is it important to simply acknowledge and not react to anything at all? (Reaction in the sense move a little...)

What is right mindfulness? Not reacting and simply acknowledging? What if it adversely affects my legs by not moving and possibly getting bitten by the mosquito?
I was being told that we don't practice to become experts on sitting. So if you  have intense pain, go the middle way. Acknowledge pain, acknowledge that you want to move, try to overcome the pain a bit, acknowledge unbearable, and after a few moments, if necessary, move the body mindfully (not automatically, in reaction), and carry on particing, again in still.

It is important to simply acknowledge, and even if you have to react to unbearable pain, acknowledge first, and react as if in slow motion. If you have pain and then worries about your legs, acknowledge fear (and let it go and go back to breath), if you fear mosquito, acknowledge fear etc.

If you become aware of an automatic reaction (movement) which has already taken place, acknowledge that awareness, acknowledge your thoughts about it (self-judgement, or whatever else arises) and go back to breath.

Middle way. If your meditation hall is on fire, acknowledge smelling, seeing, fear, then get up mindfully and take care of survival of yours and your fellow meditators.

I find the question you linked to be a very good explanation.