How can linear oceanic ridges (like the East Pacific Rise) be explained by single point mantle plumes?
Hot spots and ridges are two different things.

Hot spots stay in one place while the earth crust above them drifts and thus create a line of dormant volcanoes, they are a point features so to say.

Ridges are places where the mantle convection wells up and spreads in two directions, gently pulling the earth crust with them and creating new crust where the magma wells up. Ridges may be described as linear features.

So, without convection and thus without ridges, which form new earth crust, the hot spots wouldn't create a chain of volcanoes.