Let's analyze each statement step by step to identify the incorrect ones:

(A) Dinitrogen is a diatomic gas which acts like an inert gas at room temperature.

This statement is generally correct. Dinitrogen ($$N_2$$) is indeed a diatomic gas and is very inert at room temperature due to the strong triple bond between the nitrogen atoms.

(B) The common oxidation states of these elements are $$-3,+3$$ and +5.

This statement is also correct. The group 15 elements commonly exhibit oxidation states of $$-3$$ (in nitrides), $$+3$$ (as in $$\mathrm{NH}_3$$), and +5 (as in $$\mathrm{HNO_3}$$).

(C) Nitrogen has unique ability to form $$\mathrm{p}\pi-\mathrm{p}\pi$$ multiple bonds.

This statement is correct. Nitrogen has a unique ability to form $$p\pi-p\pi$$ multiple bonds, which is why it can form a triple bond ($$N_2$$).

(D) The stability of +5 oxidation states increases down the group.

This statement is incorrect. The stability of +5 oxidation states actually decreases down the group due to the effect of inert pair effect. For example, bismuth in +5 state is less stable than nitrogen in +5 state.

(E) Nitrogen shows a maximum covalency of 6.

This statement is incorrect. Nitrogen can exhibit a maximum covalency of 4 (as in $$\mathrm{NH_4^+}$$). It cannot expand its octet to show a covalency of 6.

Based on the above analysis, the incorrect statements are (D) and (E). Therefore, the correct answer is:

Option B: (D) and (E) only