1. The melting point increase down the group due to increase in atomic size. However, the unexpected decrease in the melting point of Sb and Bi is due to that tendency to form 3 covalent bonds instead of 5 covalent bonds, due to inert pair effect. As a result, the attraction among their atoms is weak and hence their melting points are low. Because of large size of atoms, Bi has still weaker interatomic forces than Sb. So Bi has low melting point than Sb.
2. i even have that doubt
3 & 4 i accept nithya's answer
