You have to keep in mind that when your body decays everything in your body gets weaker. Bones need nutrients just like everything else in your body to function properly. You're right though, in the very beginning stages it would be a lot harder to kill them with a hammer.

You can’t just accept some rules of reality and toss others. If you think the skull is going to decay (which doesn’t make much sense to begin with) and soft tissue or muscle won’t- then well that’s a whole other problem. I’ve worked on cadavers- I know what I’m talking about, and trust me, the skull is just as it would be on a live person.

That's not exactly what I meant. A hard blunt weapon can still crack the skull open with one good hard hit or multiple.(but yes less effective) A screwdriver would be harder to execute. I would assume you need to stab it through their eye or ear, which, if they already see you can be a big problem.

I’m sorry- but you’re just plain wrong. That’s not how physics works. Using a handheld hammer, a single blow to the skull would do little more than fracture it- you will get no penetration from it- in live individuals, the most damage you can expect to the brain are bleeds and maybe the odd fragment going deeper. A screwdriver is far more effective at penetrating the skull, even say the parietal or temporal bone (which is probably the best to go through since it’s the thinnest).