Can hanger extender be used as replaceable hanger? One advantage of replaceable hanger for rear derailleur is that you can replace it (opposite to fixed hanger). I have fixed hanger (or fixed frame) and although it is a steel frame I started wondering if I could add hanger extender as a protection tool. One point I can see it might fail to protect anything that extender is pretty short (comparing to hanger), it is more solid and there is no really a point where it could bend. So would it be an overkill (over-protection) and it won't protect anything or is it actually a good idea?

I doubt it. There is a good chance that the extender is sturdier than the fixed derailleur hanger, in which case it would add leverage, making a bent or broken hanger even more likely.