As for the worshipping, you know the answer: anyone can join. It depends on the game world, if the deity of Nature is 'Elven-only', or is not bound to any race. Naturally, Elves started it all, and feel as a kind of 'sponsors' of the place. One has to ask himself, if they stay that friendly if this place becomes popular, and lots of old people stream here in to die in peace...
I don't quite see the place as one Temple (though some locals may call it so). This forested area is also not "closed" in any way, though "gates" of sorts might be built.
I have started to think more about the actual history of the place. The reason of the gods visit (or the true reason of this phenomena - it sounds like the best explanation to the locals...) escapes me still, but might be adapted to world history, or an appropriate legend might be created.
An interesting option might a wild-magic area of sorts, influencing natural life mostly. The Elves thought this a holy place, and tried to dampen the wildest effects to a part, to be able to survive and worship here. And it was something in their magic, or more in their psyche, the way how they see Nature, that created the duality of this place!
Back to the history of both temples:
The Temple of Peace was from the beginning liked for its athmosphere. The peaceful animals, and the beauty calming your soul, no wonder a Temple was built here, or partially grown.
Leaves and stones, flowers and trees, somehow it manages to look different and beautiful from every angle. Every day changes something, be it the little flowers, or another tune of the birds, or the sky giving everything a different hue.
The Temple of Survival ended up to be the rougher side of it all. Wild, gnarled trees grow menacingly around, and seem to prepare for an attack on anyone. Dangerous creatures, predators of all kind seem to like this place and do roam about. Initially, the place was shunned, though it became a test of bravery for some foolish youths. Then came Naiser.
Naiser was a mercenery of some kind, getting older and too weary of all the useless conflicts he had been in. Hearing rumours about a wonderful place to find peace, he found, for a moment. But not even the almost-eternal spring did not have the effect he wished for. Despairing, he chose the other location, the one less talked about, to find death and a last challenge to fight. A cruel place it was, but the skilled warrior survived for some time, and found his will to life returning. With much labour he built a home, and over time many came to be tested, some surviving, some not. And a few stayed, and built other parts of what is now known as the Temple.
As for the looks, it is more a rough fort than a shrine, with massive stone walls and strong doors, but surprisingly open to Nature otherwise. Naiser is long gone, dieing in fight as he wished for. Others have walked since in his footsteps, and the Temple is known to hardened warriors as a good training place.
So both Temples are artificial creations, though they fit into the respective themes (much better than I could describe), either because of their creators striving, or from the background influence.