MSA:
Grammar: 5
Arabic grammar is woefully complex. The genders are hard, the plurals are hard, inflection is hard, verb conjugation is hard. *Shudder*
Lexicon: 5
Definitely a 5, you've gotta memorize gender AND plural for every noun you learn because of that stupid broken plural. Then there are a handful of nouns that'll take the dual. Few internationalisms.
Pronunciation: 5
Pharyngeals. That's all I gotta say.
Regularity: 4
It's really a relatively regular language as far as the verbs go, you've just got to learn all the rules, then all the exceptions to the rules. But the exceptions have rules themselves, so it's not impossible.
This is exactly why I hate the number system. ¬.¬
There's a lot of regularity in everything you mentioned (most nouns that are CaCC in the singular are CuCuuC in the plural, CaaCuuC in the singular are CawaaCiiC in the plural, those that are CaCaC take ’aCCaaC or CiCaaC in the plural...; all nouns take the dual and it's super-easy; gender is incredibly easy to figure out...), and all languages have nasty allophones to get used to. I really don't think the language deserves a 5 in all categories. :/