One positive (or at least non-"negative") thing I will say is that the video does draw attention to two interesting questions:Agreed, not all criticism is meant to me negative but if your only reaction/comment on something is purely on what you DON'T like about it, it will naturally be perceived as negative. A critique should be well-rounded and balanced. Not everyone will assume that you like the video if you do nothing but complain about the things that you disliked.
1) What was the extent of Mande influence in early Gao, and did a branch of the Mande peoples play an important role in the foundation of the state or in the ethnogenesis of the people that ruled the state?
I think this isn't easy to answer. The idea, elaborated most thoroughly by Lange, that there was a substantial Mande role/influence of some kind, is actually entirely plausible, and is not some sort of far out theory at all. Although the evidence in support of this idea isn't something concrete, it may really be the case that there was a significant Mande contribution, and the signs of this possible Mande factor in Gao that Lange drew attention to in his articles should not just be dismissed. Even if we can't speak of an actual conquest of Gao by Ghana, or of Gao as being incorporated into Ghana's empire in any way (the actual contemporary primary sources only ever indicate that they were separate states) as Lange suggested in one of his articles, it is still a possibility that the early rulers of Gao were of Mande origin, like those of Ghana. The fact that the Askia dynasty of the later Songhai empire was of Mande origin is also a reason that this possibility should not be ignored when considering the origins of the first dynasty of the earlier Gao empire.
There is also a very interesting article by Paulo Fernando de Moraes Farias, "Bentyia (Kukyia): a Songhay–Mande meeting point, and a “missing link” in the archaeology of the West African diasporas of traders, warriors, praise-singers, and clerics" (2013), in which the author discusses the Mande role in Kukiya during the time of the Songhai empire, and also suggests a possible Mande origin for the Sonni dynasty of Songhai.
The full article is accessible here:
Bentyia (Kukyia): a Songhay–Mande meeting point, and a “missing lin...
2) What was the real extent of the political influence/authority/control of Gao in the 800s, 900s, 1000s, etc.?
This question would probably be even more difficult to answer. The information about Gao provided by al-Yaqubi and al-Idrisi suggests that it probably did have considerable political authority/influence well beyond its immediate area for centuries, long before the rise of Songhai. How far exactly that authority went will probably remain mostly a mystery, but if the location and extent of some of the polities that it was supposed to have authority over are ever determined, then a more accurate map of early Gao could be made.