>Hebrew was literally synthesised a century ago.

I had heard somewhere that much of the vocabulary of Modern Hebrew consists of loanwords from Arabic. Is this correct and if so, would it mean that the "cleanliness" of the language is more a reflection of Modern Standard Arabic?

Apologies in advance if this is seen as some falsehood or if it's a sensitive topic.

No, that isn't true. Hebrew has taken a lot of Arabic words but not the majority. It has also taken a lot from Yiddish (as you'd expect) and certain modern words which are common across Europe.

No idea. But vocabulary and grammar are mostly orthogonal.