Which language for Arabic World?

Can someone from, say, Morocco understand a fellow Arabic speaker from Iraq?

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No they can't, it's really different. The language of communication is Classical Arabic, but then you need to go to school and study to learn it.
Between morocco and egypt, their Arab are not so different, but not between Dubai and morocco .
You can speak french in Algeria, Morocco and Tunisia but you will need Arab too.
So french and classical arabic are good choices, with english of course.

How different are the varieties of Arabic really? Is it something like the distance between French and Italian? Or is more like the distance between French and Romanian? The Living Language Arabic course has examples of different kinds of Arabic.

I found the discussion (civil, unbelievable) about conlangs very interesting, but going back to the discussion of the difficulty of choosing a lingua franca, I hope someone will be able to give an opinion on the varieties of Arabic.

Found the post about Italian/Spanish/Interlingua interesting. Anyone have first hand experience speaking Interlingua to a Spanish or Italian speaker. I think it sounds like a dialect of the latter. Thoughts?

Yeah, I've spoken Interlingua to a Spanish speaker, and wrote it to an Italian speaker. And if you know some Spanish or Italian, you can even throw some in and speak a hybrid Spanish-Interlingua!

What about using Interlingua with Romanians?