Not so. The Quran assumes the validity of the Torah and the near-truth of the NT.
Wrong. The Qur'an explicitly describes the distortion of both documents in the hands of men and their consequent unfitness as reliable sources of spiritual guidance. Religious texts and doctrines outside of the Qur'an and Islam are peripheral. I suggest you read the Qur'an and understand it before making claims as to what it does and does not say.
Muhammed claimed to be the next prophet of the god of abraham and, like jesus, cites the Torah as that which provides the authority for his teachings
Incorrect. The authority of his prophethood was derived from his comprehension of the divine; not from the Torah, the Gospels, or any other such text.
How do Muslims explain the intertestamental period suddenly ending at the very odd 630AD when Prophet Mohammad made his absurd claims?
With Jesus Christ the Intertestamental period ends after an interlude of about 400 years, which is consistent with the symmetry of Hebraic events. 400 years from Moses to the Temple, 400 years from the Temple to the Babylonian Capitivity, 400 years from the Captivity (and intertestamental period) to Christ.
Since that Captivity there have been NO PROPHETS.
Except those prophesying Christ and His arrival. Why would God wait, in this wisdom, 400 years, 400 years, and then 1,000 years?
That doesn't make any sense.
Not to say the 400 years have any "reason" to it, but at least they are symmetrical.