MTS is erasing my items.

no?
you agree in like 9999 places (installation EULA, registration EULA, first-time ms use ToS, nx charging ToS, paypal's ToS, etc.) that nexon can do anything they want with your character\nx cash and that once you bought it, the money is not yours anymore and you cant get any refunds.

Speaking of which, I know Nexon keeps changing their ToS. Does that mean Nexon's ToS and EULA is invalid, due to it being different from the one you originally agreed to?

The bolded isn't true, it's applied when you charge the NX. That's why you can still go to the store and ask for a refund, but if you already charged the money and you refund it at the store, then they'll bounce back and Nexon can ban your account. What if you try charging and the money that was supposed to be there got rolled back (then it's technically not NX)?

What if you spend it from PayPal (in which Nexon withdraws from the PayPal but your cash items get rolled back)?