Dear brother, islam and kufar have two opposing basis. If one leads to peace then other leads to troubles therefore disturbances and even wars. Islam is a complete ideology for social, political, cultural, and economic systems, structures, procedures and practices. This is why Allah does not accept any way of life other than the way that leads to peace.
The point here is that islam gives a whole different meanings to all that people say or do ie it gives things its own context or if you like islamic basis. In this context all things harmful for humanity are unlawful therefore all that benefits mankind is lawful or legal.
Allah therefore tells muslims to enter islam fully not partially. The adulteration of islam and kufar is not acceptable at all. It is not ok to drink a whole glass of water with little drop of urine.
The point is that muslims ought to say and do things for peace whereas kufaar say and do things for their own ends.
There are lots of similarities in our actions but that is natural eg muslims are people but so are kufaar, muslims talk and so do kufaar, muslims eat and so do kufaar, muslims marry and so do kufaar. muslims have kids and so do kufaar. Whatever either of them say or do it has different purpose and context.
Divine set morals and ethics or values and formulas remain the same always only and only ideas, words and deeds of people change for better or for worse.
In real life due to incomplete sense of things we humans do come across situations when relevance becomes a problem eg choice between bad and worse. So long as we make a choice for the good of others, we are ok because we are not Gods but humans with limited knowledge and power hence not free of possibility of human error.
As for marriage is concerned, it is a choice of individual adult people who they choose to marry but there is no room for coercion of any sort in islam. If other arrange it and both the involved people are ok with it, it is fine. This is islam’s general rule. If a woman of 21 years chooses to marry a 100 years old man with his agreement that is her choice and should be respected.
The only problem is when is a person accepted as an adult in any given society? Here we have two points to take into account a) the true indisputable age and b) just any age any particular society chooses for legal reasons or its customs. Indisputable age cannot be more than 21 and less than 18. The rest is up to society as whole to do as they see fit. Only ignorant people will do things senselessly and many do because a lot of people are not any better than animals. However whatever people did wrong divine messengers were never part of it and that is because once they had guidance from on high they lived up to it.
Coming to attribution of child bride to the prophet, it is not right because at the age of six a child has no real sense of anything much. One has to see how many 6 year olds will be allowed to become prime ministers in a country or chiefs in tribal societies or be accepted as soldiers? Looking at the nature of mission of prophets, would they need mature partners or childish ones? In many ways this thing makes no sense. This is not a matter of exception because prophets cannot break moral norms because they are for all times. How can a prophet who comes to tell others don’t remain stupid and don’t say or do stupid things do them himself?
As I already explained, there were ample conspiracies against prophets in order to ensure their failure. Disinformation and misinformation was always the best tool for enemies to get their enemies by. So reports that attribute certain things to the prophet in order to discredit him are unacceptable no matter what.
Not only some might have attributed things to discredit the prophet but some might have attributed certain things to the prophet in order to gain access to certain things for themselves. You see if a mulla has tendency for children, he could tell ignorant people around him look people even the prophet married a child so why can’t I. Are you stopping me from acting upon sunnah of the prophet. So there are various reasons we should not go down that way.
It is best that we stay away from things that could easily lead to disputes whereby we ourselves could end up misinterpreting islam and therefore misrepresenting it.
regards and all the best.