First off, the only power the gods have that they’re allowed to use is literal skills like smithing or winemaking and granting Falna. That said, while Haphaestus’ smithing skills without the Arcanum are theoretically achievable by a mortal they are the pinnacle because the gods are actually very skilled with their crafts if they have one. Other than that, they are restricted to the same power as a mortal. In fact, they’re physically below level 1 with their powers sealed.
Next, it’s not that mortals can’t kill them they just can’t be perma killed. Mortals generally are resistant to harming a god but if they are fatally wounded their seal comes off and their power, the Arcanum, auto heals them. However, since the whole thing is a game to the gods, using the Arcanum is cheating, so they are forcibly sent back to the upper world and can not return again. Gods generally kill other gods because it is a mental burden to most mortals to do so. It’s basically a sin because despite the weirdness of most gods, they also ended a pretty dark period in history by containing the dungeon, thus they are generally respected.
As well, Falna are sealed when the god who granted it returns to the upper world. They return to the level they were without the Falna. After that, it can be unsealed and reactivated by converting to another familia. Comparatively, if the god is alive but you want to change familias you must get their permission before you can convert it to another god’s Falna. Power levels are retained though.
