Why a twelve-gun 15-inch Nelson was not the likely alternative

Would an all-15-inch treaty alternative have been a Nelson-style ship with twelve guns?

Probably not. The 35,000-ton limit points toward a nine-gun arrangement even though the proposed 15-inch/50 would fire a somewhat heavier shell.

The follow-up asks whether the Royal Navy could simply place twelve 15-inch guns on a Nelson-style hull. That was possible in the abstract but was not the arrangement the navy was likely to pursue. The 35,000-ton treaty ceiling explains the preference for a nine-gun solution. The proposed 15-inch/50 would also use a somewhat heavier shell than the older weapon. Changing calibre does not remove the displacement choices among gun count, protection, machinery, and hull size.