If either target is illegal by the time Searing Blaze resolves, it still deals damage to the other target.

You must choose two targets as you cast Searing Blaze: a player and a creature that player controls. If you can't (because there are no creatures on the battlefield, perhaps), then you can't cast the spell.

Once the spell resolves, having a land enter under your control provides no further benefit.

Having more than one land enter the battlefield under your control this turn provides no additional benefit.

Whether you had a land enter the battlefield under your control this turn is checked as this spell resolves, not as you cast it.

The landfall ability checks for an action that has happened in the past. It doesn't matter if a land that entered the battlefield under your control previously in the turn is still on the battlefield, is still under your control, or is still a land.

The effect of this spell's landfall ability replaces its normal effect. If you had a land enter under your control this turn, only the landfall-based effect happens.