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If the targeted player becomes an illegal target by the time Suffer the Past resolves but the targeted cards don’t, you still exile the cards and gain life. The player won’t be lose any life, though.

Suffer the Past’s effect causes a single life-gain event, not a number of separate ones. This matters for cards such as Cradle of Vitality.

The life that’s lost and the life that’s gained are based on the number of cards that are actually exiled, not on the value of X (in case any of the targeted cards somehow leave the graveyard in response and thus can’t be exiled this way).