1)Neutron emission is not a possible mode of decay for K(40,19).Why?
I think it is because the neutron to proton ratio for potassium(40,19) is almost unity(more precisely N/P=1.1). So, neutron emission is not a mode of decay to gain stability. Is it right?

 

Why? It is the strong force that governs reactions in the nucleus and it operates over very short distances. In a K(40,19) nuclues the energy is not there for the emission of a neutron.

The wikipedia article mentions only Helium-5 and Beryllium-13 as examples of neutron emission, and then says that by definition the decay of Helium-5 is a case of alpha decay. I guess they think a really PO'd neutron ejected the Alpha particle. That's a hard one to get your arms around.
http://en.wikipedia.org/wiki/Neutron_emission