However, in reality even the full 256 bits range and beyond work.
For Example, using 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF, we get the address 1PRWyFKTsQSJaUdX9VKgQNw8JERPw2kMFm that is valid (even transacted before).
No, they don't work because they are technically invalid. You are just using a tool that decided not to show you any error or warning message that the key you gave it is out of range, and instead handles it silently under the hood.
As far as I understood, in ECDSA calculations we use %N, therefore we loop, and there is also a prviate key < N that also gives the same address 1PRWyFKTsQSJaUdX9VKgQNw8JERPw2kMFm.
am I right ?
In ECC we are working in a finite field so the operations are always modulo m.
Any private key > N is invalid, you can modify its value to make it valid. One way is to compute is mod N.