strength not to prevent unscrewing. The through hull fitting is vulnerable to impact damage. The risk increases with electrolysis. I think there is an ABYC requirement for impact resistance, not that I am a big fan of ABYC.

I have heard stories of Marelon handles breaking off but I have never experienced it. A handle breaking off is far less impact than a through hull breaking off.

A backing plate under the seacock is like a washer under a nut plus it levels off a curved hull to make the nut seat better.

Just because the part of the ball valve you see shows no corrosion don't fool yourself into believing it could not be rotting out on the inside. It only means that the part you see has net been wet with salt water. I can show you plenty of proper bronze fittings discolored from salt water exposure. My Groco raw water strainer is green streaked from minimal salt water exposure.