Is the 22.5% tax on 50k salary including social security taxes?

If you’re tax resident in two countries the tax free allowance in the other country doesn’t matter. Unless a tax treaty changed how the taxes are distributed.

Yes, and the best thing is that you don’t have to pay any tax!

I think there’s a confusion about the terms, is a “mailing address” in NL an official name for the address where you’re registered?

I use it also. For 3 years now abroad, still working.

What do you mean by your country not allowing you to have a mailing address? Of course you can use a mailing address in any country, how could they stop you from dojng that, or how would they even know?