Shell fuzes behind layered battleship armor

Would an armor-piercing battleship shell that penetrated the main belt explode before reaching an inner plate?

Probably, but the exact result depends on fuze delay, impact angle, retained velocity, and the geometry of the inner armor.

The outer 300 mm belt appears substantial enough to initiate the shell's fuze, making detonation before the inner plate likely. The result depends on fuze design and target scale: even an 18-inch shell may detonate inside a small escort if its delay matches the transit time. An armour-piercing projectile can also pass entirely through a lightly built target and explode in the water when its delay assumes a battleship's citadel depth. A reliable judgment therefore requires the particular projectile, fuze, retained velocity, and geometry rather than armour values in isolation. Exact predictions would require technical calculations or a simulation beyond the expertise claimed in this answer.