I want to know how is the best arrangement for writing a PRL "100-word compelling justification"?? What factors should be mentioned and what should be avoided?
This is partly a matter of critical thinking. The more points you make the less words you will have on average for each. The more important points should be made first with more words. So apart from the relative importance of your points, which I cannot help you with, there is also an optimum number of points to make to maximise the compellingness of your justification.
Ask the question: Why must a busy PRL reader read your article? What text should the editor put in his / her editorial to encourage his / her readers to read your article?
Ignore all dead-ends and negative results. Emphasise how your interesting article helps to advance our state of knowledge and affects practice. Be specific. Explain what is novel. Waste no words. Not even one.