Learn English – “Many lost their life” or “Many lost their lives”

Plural, because we are, in fact, dealing with eight little backs. And a back is a back, no matter how small. A child has a back, but children have backs.

I'm sure others will back me on this.

Everyone else's life would be correct. In this case everyone is a singular pronoun. (See this explanation of everyone being singular.) Else is just a determiner used after everyone.

Allow me a digression that I think you might find helpful when thinking about whether to use life or lives in similar sorts of construction. The most memorable explanation of whether to use a plural or singular word that I've ever encountered comes from the book Words Into Type (p. 357). Under the heading and subheadings of Nouns, Number, Singular with a plural possessive there is this clear explanation:

To avoid ambiguity a singular noun is often used with a plural possessive when only one of the things possessed could belong to each individual.

• Manufacturing helps many people in the smaller cities to earn their living.

• Forbes knew most of them by their first name.

• Some of them could not pay their rent.

• They eyed each other furtively and cursed beneath their breath.

Similarly:

• Think of the last name of five pupils in the room.

• The steam line ruptured, causing the death of seven longshoremen.

• They doubled their efforts to discover the identity of two men who struck a man with their automobile and then fled.

Care must be taken not to apply the rule to the wrong noun.

Wrong: It is pretty clear that the smile on the face of the delegates, whenever they look at each other, is not a sincere one.

Right: ...on the faces of the delegates ... [ Smile is the noun the rule applies to.]