You can sacrifice any Eldrazi Scion to activate Drowner of Hope’s last ability, not just one created by Drowner of Hope.

You can’t sacrifice the same permanent to pay two different costs. Notably, if you sacrifice an Eldrazi Scion to activate the last ability of Drowner of Hope, you won’t add to your mana pool.

Cards with devoid use frames that are variations of the transparent frame traditionally used for Eldrazi. The top part of the card features some color over a background based on the texture of the hedrons that once imprisoned the Eldrazi. This coloration is intended to aid deckbuilding and game play.

A card with devoid is just colorless. It’s not colorless and the colors of mana in its mana cost.

Other cards and abilities can give a card with devoid color. If that happens, it’s just the new color, not that color and colorless.

Devoid works in all zones, not just on the battlefield.

If a card loses devoid, it will still be colorless. This is because effects that change an object’s color (like the one created by devoid) are considered before the object loses devoid.

Eldrazi Scions are similar to Eldrazi Spawn, seen in the Zendikar block. Note that Eldrazi Scions are 1/1, not 0/1.

Eldrazi and Scion are each separate creature types. Anything that affects Eldrazi will affect these tokens, for example.

Sacrificing an Eldrazi Scion creature token to add to your mana pool is a mana ability. It doesn’t use the stack and can’t be responded to.

Some instants and sorceries that create Eldrazi Scions require targets. If all targets for such a spell have become illegal by the time that spell tries to resolve, the spell will be countered and none of its effects will happen. You won’t get any Eldrazi Scions.