Interesting quote, but you didn't cite the source.
So I looked it up. You are quoting from a Pervezi source, a book published by the Tolue Islam sect authored by Dr. Syed Abdul Wadud.
Nevertheless, the quote itself says "in those days the idea of Imam Mahdi was in vogue". So your argument that Abu Muslim al-Khurasani invented the concept of the Mahdi is refuted by your own source. The truth is that the concept of the Mahdi is that he is a descendant of the Prophet through Fatima (peace be upon them both). Abu Muslim was not a Fatimi so obviously he didn't "concoct" the idea of the Mahdi.
It is possible that the Abbasids fabricated some narrations about black flags coming from Khurasan in order to portray themselves as the Mahdi's army, but it is equally possible that they adapted the black flags in order to fit the already existing narrative based on Islamic traditions that can be traced back to the Prophet (Sallallahu alaihi wasallam).
You have to read the context of Sura 43: 61 in order to realize that "he" is a pronoun that is referring to the Messiah. Begin reading from Sura 43: 57 to appreciate that it is talking about the Messiah.
As for Sura 61: 14, it is not necessarily talking about an army or overpowering the "enemy" by force of arms. Historically speaking, the disciples of the Messiah of Nazareth never waged a war or engaged in armed conquest. Therefore it is more likely the "triumph" spoken of here was a spiritual triumph.
