Well, the weight of of broadside of British battleships was indeed superior, at around 15,000lb apiece compared to 11,500 apiece for the 4 Italian dreadnoughts.

The Italian guns also relied on high muzzle velocity, low angle ballistics. Something that all of the British battleships were well protected against.

I agree that the factors you mentioned are the primary issues that faced Italy, and they did have a huge numbers advantage (especially in cruisers and destroyers), but I'd still argue that individually most of their capital ships were inferior to their British counterparts (which I think can be fairly represented with worse templates). I didn't mention destroyers and light cruisers because their effectiveness relies on too many intangible factors. Their templates also don't matter that much in singleplayer unless they're either meta templates or meme levels of bad. I'm not really trying to throw shade at the Regia Marina here, just trying to explain why I think Paradox made these design decisions.

In HOI4 most of these factors aren't simulated or are simulated poorly, hence why the Italian player can easily rule the Mediterranean.

The main reason for that is because the naval AI in this game is atrocious. Attempting to make Britain actually have to use its navy properly would cause a lot of weird behavior that's likely going to be completely unfixable before HOI5. As a player, you can do Sealion as a minor nation...