After you activate Soulfire Grand Master’s last ability, if the next instant or sorcery spell you cast from your hand doesn’t resolve, it won’t be put into your hand and the effect won’t be applied to the next spell you cast.

A spell that instructs you to exile it (for example, Temporal Trespass) won’t be put into your hand because it’s not put into your graveyard as it resolves.

An instant or sorcery spell with lifelink causes its controller to gain life only if it’s the source of any damage that’s dealt. An instant or sorcery spell with lifelink that causes another source to deal damage won’t cause its controller to gain life.