One of the explorer's comments on the holy power of Divine Dragon Falls:
[Explorer]
Ancient lore reveals that
the Dragonites knew of
certain land formations
they called Power Spots.
There, they are said to
have used the natural
energy of the planet to
generate Elements.
You need the two Tears
and the blessing of all seven dragons to make the Chrono Cross. Along with a "power spot" that the Dragonians used to make elements, why
wouldn't Divine Dragon Falls be able to make the Chrono Cross? Steena even comments on the place's power.
Steena:
By the Great Dragon Gods!!!
The sacrarium of the Divine Dragon Falls
still exists...and it seems functional!
It's just as the legends of old said
it would be! What a blessing!
Steena:
Utilizing its power, we just may be able to create
the '"Chrono Cross"' here!
Steena:
Try placing the two pieces of the Dragon
Tear... The Tear of Hate and
the Tear of Love...on these stands.
Now, if you go to Divine Dragon Falls before going to Chronopolis (with Steena in your party), the six dragon relics Serge has gathered from the various Dragons float along with the two Tears, but won't form the Chrono Cross. Steena comments on the seventh element/dragon then:
Steena:
Great Dragon Gods!
We are yet unable to
utilize the altar...?
I know that we need
the six Dragon Relics
that offer up divine
supplication...
Could 'that' legend be
true... Is there a
seventh dragon, and a
seventh Element...!?!
Steena:
When the time is ripe,
we shall be able to
generate Elements here!
So to answer your first question, the Dragon Tears themselves are not the only ingredients necessary for the Chrono Cross. And it was the planet's power that unified the shattered Tears, because Divine Dragon Falls is one of the "power spots".
As for who put the pedestals there? The game does not say who, but...Steena obviously has heard of the sacrarium, so it was written in Dragonian lore somewhere.
The only instigator I can think of would be FATE, who used the power of the Dragon God to create and terraform the islands of El Nido. Belthasar, as the curator of Chronopolis, could very well have programed in a directive to FATE to create Divine Dragon Falls, knowing that one day Serge would need it to create the Chrono Cross.