I think you are right about Mayer-Vietoris. It gives the isomorphism $H_n(sum)=H_n(M_1)\oplus H_n(M_2)$ and since homology classes can be represented by surfaces that don't hit "gluing disk" we get that the intersection form of the sum $Q=Q_1\oplus Q_2$.
If a manifold admits a smooth (or even PL) structure then KS vanishes.
In dimension 4:
I think that K-S invariant is independent of the intersection form if it is odd. If the form is odd then there're two homeomorphism types of manifolds (Freedman) that are distinguished by the $KS(M)\in H^4(M,\mathbb Z_2)$.
If the form is even then KS(M)=(1/8)signature mod 2.
If a manifold admits a smooth (or even PL) structure then KS vanishes.
