Down the group due to increase in size the strength of metallic bond decreases and so, melting point decreases.

Atomic volume of K is larger as compared to Na. Increase in atomic mass does not over weigh the effect of increase atomic volume in case of potassium.

They have low ionisation energies because of their larger atomic size. The heat from the flame excites the outer most orbital electron to a higher energy level. When the excited electrons come back to the ground state, there is emission of radiation in the visible region. Hence, salts of Li to Cs impart characteristic colour to an oxidising flame. (of Bunsen burner).