It should be pointed out that internal lines in Feynman diagrams do not ”go” in any direction. As such, the labelling of ##W^+## in the diagram is dubious as the diagram equally well represents the other time ordering where one might say that it is a ##W^-## going from the right to the left. In fact, people often take Feynman diagrams way too literally as if the lines were representing the worldlines of little balls moving around. They are nothing but (very very useful) graphical representations of terms in a mathematical series expansion.
When it comes to charge conservation, you can always draw an arrow on a line representing the charge flow and use charge conservation at each vertex. (You can do this for any charge, not just electric charge.)
You are correct that it must be a lepton. What charge must it have based on the above?
