Who is blind, but my servant? or deaf, as my messenger? God's original "servant" and "messenger" to the nations was his people Israel. It was only through their default that he needed to send another and truer messenger.
He now asks, having regard to their opportunities, who are so blind and deaf as they are? The object of the question is to wake a feeling of shame in the hearts of those who are not shameless among the Israelites.
That I sent; rather, whom I will send. Israel's mediatorial office was not yet over. They were still, for above five hundred years, to be God's messenger to the nations. As he that is perfect; rather, as he that receives reward from me (see Proverbs 11:31; Proverbs 13:13).
The word used is connected etymologically with the Arabic muslim (our "Moslem"); but it does not appear to have had the sense of "surrender" or "submission" in Hebrew.