Concerning your number 1 question, NO, back to sleep is the recommended sleeping position by paediatricians

You don't need to teach a baby to sit... The baby will sit on his own... Most children sit by 5 to 6months.

He/She will sit by herself usually from age 5 - 6
months.

At 3mnth u should strt teaching the child with support cos the child il not b able to control his body movement and by 4th month the child should b able to sit up on his own for a period of time cos by the 4 month his back and neck muscle should b getting firm though he il still need support bt it also depends on ur child developmental capacity.