Questions regarding meditation posture and movement

I was trying sitting cross legged and back straight during Anapanasati meditation. I have looked at this question -> https://buddhism.stackexchange.com/questions/3937/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?