Do Fender Replacement Necks Need Fretwork?

Exactly. In my experience with a few Warmoth & AllParts necks, none of them NEEDED any fretwork. The AllParts probably would have benefited from some minor touch-ups. One of the Warmoth necks I've had is near perfect out of the box -- I'd put that guitar up against any American Fender in terms of playability and feel.

Are Strat necks interchangeable?

All necks are interchangeable to a point, but all neck pockets are different. DO not expect a new neck, or a different old neck, to fit perfectly.

Are Fender necks universal?

Check the Neck Spec

Unfortunately, they are not universal…even among Fender products. ... Most guitar bodies and necks employ the Stratocaster shape, which is rounded at the end. However, bodies and necks from the Telecaster family use a heel that is squared. It's always best to match these shapes correctly.