how risky is it to brick my Wlan Router when i replace the firmware with the Merlin one? how can i go back to the original Firmware, do i lose Warranty when i do this?
Merlin is in close contact with Asus and his version is approved by Asus! So there is no risk over the original Asus version!
Of course you need to follow the recommendation about a NVRAM/factory default reset plus a power down AFTER the update/change of firmware as described here.