How a third propeller shaft could fit the Nelson class

Would a third propeller shaft in the Nelson class need an awkward angle?

No; the internal layout could support a short shaft on a nearly direct line.

The proposed shaft must be judged against the position of the boilers, turbines, existing shafts, and rudders. In the illustrated layout there is enough longitudinal depth to keep the additional run comparatively short. It could emerge close to a direct line instead of requiring a severe bend. The answer makes machinery arrangement, not the view from outside the hull, the starting point for evaluating the modification.