Q: Is it worth trying to get item level 70 Reaper of Souls Hellfire Ring?
A: Of course it is worth. Instead of giving 35% XP bonus of the current item level 63 Hellfire ring, the level 70 hellfire ring would give a 45% experience bonus. And it would cast a fire ring for 200% weapon damage which is really cool. With the enchant system, it would be easier for you to get the stats you want.
Q: Can I equip the legacy HR and the ilvl 70 Hellfire Ring both in the expansion?
A: No, you can not. You can only equip one unique items at once. But you can have one ring on your follower to get the bonus.
Q: Will current keys useful to craft ROS ilvl 70 Hellfire Ring?
A: No, you can't . Diablo 3 keys are
The Key of Destruction
The Key of Hate
The Key of Terror.
But level 70 HF ring crafting keys are
Key of Bones
Key of Gluttony
Key of War
Key of Evil
The new keys still drop from the Keywardens. And they have the chance to drop random key from other Acts as well. So your current hoarding keys can only craft item level 63 Hellfire Rings.
Q: What do we know about the Inferno Machine of Evil?
A: This is the inferno machine which would introduce the fourth group of Uber Boss. The new Uber boss is Diablo which would summon Skeleton king and other Uber bosses. Diablo would drop Key of Evil which is one of the fourth ingredients to craft the New ROS Hellfire Ring.
Q: What is "Strength in Numbers"?
A: This is a bonus to encourage players to play in a team instead of solo. In a four player party, you could get 30% XP bonus which would be higher you can get from your follower.