The US Attorney has agreed to recommend a sentence no higher than 18 months (assuming Trevor has no prior criminal record).
No, the prosecution agreed to recommend a sentence at the minimum of the range if the offense level is at least 15. At level 15 the minimum sentence is 18 months, but the judge could use a higher level with a longer minimum sentence. IOW, the plea deal sets 18 months as the shortest possible sentence, not the longest.