Gun modification in self defense?

I think you're worrying too much about it. I don't know of any law that would limit your ability to replace the guide rod in a semi-automatic pistol.

That being said, replacing the guide rod is just stupid. I don't know of any gun that comes with a plastic guide rod. There might be a gun with a polymer guide rod, but if so, its stronger and more durable than steel so it would be foolish to replace it.

Most of the common pistols on the market today will go 5,000 rounds or more before they need any replacement parts. So don't over-think this. If you're buying a pistol for self defense, spend your extra money on ammo, range fees and a training course, not on replacement guide rods and trigger jobs.

Shouldn't make any difference whatsoever. It does not change how the gun functions at all.

A trigger job might be a problem, because it usually lightens the pull and thus more likely to shoot accidentally. But the guide rod wouldn't have that problem.

Trigger jobs, loading your own ammunition, and modifying the guide rod, are all legal and quite common.

Self Defense is not really a matter of what weapon you use, it's a matter of whether or not your life was in danger.

It's never been an issue in any case I ever heard of.