The facts are more complex than that.
Unlike the stable photon, the W bosons have a negligible lifetime.
So in fact those processes listed by you that result in 2 particles are 4-particle processes.
Two particles collide, either a quark and an anti-quark giving other 2 particles, a lepton + an anti-lepton (in your exemples positron + electron neutrino or muon + muon antineutrino) or a lepton collides with an anti-lepton giving a quark and an anti-quark (in your exemple up quark + anti-down quark).
So there are always 4 particles, either quark, anti-quark, lepton and anti-lepton, or there may be both at input and at output a lepton and an anti-lepton, which includes the case of scattering through weak interactions, or there maybe both at input and at output a quark and an anti-quark, which includes the case when a meson decays into another meson through a weak interaction.
All the interactions involving W bosons are 4-particle interactions, where the W boson is an intermediate particle that exists only during a negligible time.
The 4 particles may be all at the input, all at the output, 3 at the input and 1 at the output, 1 at the input and 3 at the output or the most frequent case, 2 at the input and 2 at the output.
Like all the interactions between elementary particles all these cases are equivalent and all the variants are interchangeable by moving a particle from the input to its anti-particle at the output or vice-versa.
In most cases you need to pay attention only to the 4 particles participating in the weak interaction and not to the intermediate W boson.
The existence of the W boson matters only when you need to compute various things because this intermediate particle transforms diagram nodes with 4 edges that cannot be computed into diagram nodes with 3 edges that can be computed with the methods of quantum electrodynamics.