La-Mulana/WMG

The real ending to Hell Temple was Lameza becoming possessed by The Mother.
The Treasure and the Fan Disservice picture were just Lameza's crumbling mind coming up with a symbolic representation.
 * Not too likely, given Japanese humor. Besides, Mother isn't yet awakened when you enter Hell Temple (she's still sleeping in her shrine), so that really doesn't add up.
 * Word of God stated that the original ending of La-Mulana was Lemeza saving a girl trapped in a coffin for so long, so when they scrapped it, that ending might have been possible that they have thought about using that ending for Hell Temple and then later scrapped it again in favor of the "Treasure that must not be seen".

If Lemeza dies during the final boss, all of humanity dies.
Allow me to quote the intro of the Wiiware remake. "...This is the day Humanity was born. Then when they angered her, they were destroyed.". Fighting her is pretty sure to annoy her, and, well, note the fetus ball in the background at the end - This could be the first of a new breed of human, destined to take the place of the humans when they were obliterated. Thank god for Lemeza putting her to rest before she could do that. But if he died and the Mother remained alive...
 * While I disagree with the new human breed in the ball (It drops the 'Treasure of La-Mulana' when the final boss dies, implying it was it's true form) the rest may very well be true. And in the Wiiware remake, there are supposed to be three endings. As one of them is the original ending with better graphics and minor changes, one of the other two may be just that. Or a Nonstandard Game Over.