Misaq is for the Imam. While in theory there is a clear distinction between the Imam and the Dai, in practice this distinction has little consequence. Let us wait until Insaf Saheb makes the whole text available. Then we can make sensible comments.
It would be foolhardy to suggest that a fluent knowledge of Arabic is sufficient to create a work of poetry, marsiya or other work of art any more than a fluent knowledge of English will enable one to write a novel or a poem. But being an Imam or a Dai does not mean he cannot write excellent poetry or marsiya. One would have to be quite skeptical of any claim that disputes Sayedna's authorship of Sayyid as-Shuhada marsiya.
Sayedna having an interpreter while conversing with Arabs makes a great deal of sense for several reasons:
Firstly, Arabic is an example of a language exhibiting extreme diglossia (
http://en.wikipedia.org/wiki/Diglossia). Spoken and written languages differ greatly. Modern Standard Arabic is not suited for conversation and even educated Arabs fall back on Egyptian Colloquial Arabic to communicate with Arabs from other countries. Hence, there would be a need to choose a different interpreter in different countries.
Secondly, Bohra religious education invariably teaches Classical Arabic, not Modern Standard Arabic. Even these two differ greatly. I have a copy of the Quran with explanations of words in Modern Standard Arabic in the margins. Sayedna, most certainly, speaks Classical Arabic and not many Arabs, even educated ones, will really understand it. Marsiya, ya sayyid as-shuhada, is written in Classical Arabic. (In a waez delivered in the Yemen, Sayedna spoke in Classical Arabic. I suspect that most Yemenis did not clearly understand him but they got the sense of it. Even a Bohra with no knowledge of Arabic would get a sense of it.)
Thirdly, there is the matter of accent. Sayedna has a marked Gujarati accent when he speaks Arabic even when reciting the Quran or Duas. This would make it a bit harder for Arabs to understand him.
For these reasons, it is always advisable to have an interpreter.