Hood's armor depended on range and hit location

Was Hood armored against her own 15-inch shells?

At long range and in the right protected areas, yes; at ordinary ranges or outside that coverage, not reliably.

Armor comparisons depend on the area and depth of the hull protected as well as nominal thickness. Hood could resist her own 15-inch gun at long range in the protected areas. The same claim does not hold reliably at ordinary engagement ranges. Equal armor thickness on a battleship and battlecruiser also does not mean that the same proportion of each hull is protected.