U need to brush up on ur knowledge of prophets daughters marriages, his eldest daughter was married to her cousin from hazrat khadijas side, khdaijas sisters son , abu al aaas ibn rabi.
Prophets second daughter ruqaiyyah and umm e qulsoom were married to,two brothers utbah and utaibah who were sons of abu lahb, and abu lahb was prophets paternal uncle, its just like the marriage of hazrat fatimah and hazrat ali.
So in essence all of prophets daughters were wedded to first cousin or fathers first cousin.
Also u said "now the marriage between cousins is permissible but not encouraged" seriously is that an attempt on ur part to make cousin marriage look bad? U made it look like divorce, cause neither its encouraged nor discouraged , its all upto ones personal choice.
As of a clan with people born with weak legs. In some clans some genetic disorders do run over generation and normally even from outside those clans hesitate to marry into those familie sso that their kids dont suffer same weaknesses.
Lastly, u said the prophet spoke of marrying far and wide , was that hadees made as an argument against cousin marriage? Can u share those hadees so i can read the context
I do not need to brush up at all, rather we should both be specific regarding the time frames. Yes Zainab was married Abu al-As (her relative) and continued to be so after his conversion to Islam. Had he not converted this marriage would have been nullified. However her sisters i.e. the Prophet's daughters there was a different turn of events.
Ruqayyah
was married to Utbah but he divorced her on the request of his father, and was not Muslim. He later converted and Ruqayyah married Uthman(ra), which is widely known.
Like her sister Umm Kulthum was also married to a non-Muslim (her relative) who similarly also divorced her, and something which the Prophet(saws) underwent an insult for. He was the prophesied to have a horrible death which he did.
Umm Kulthum went on to marry Uthman(ra). Uthman(ra) himself was known as the holder of the two lights i.e. he married two of the Prophet's daughters. The earlier marriages mean nothing, aside Zainab's one, as they were nullified and can't be used in this discussion.
You asked me question if I wanted to make cousin marriage look bad, no I didn't and never have because it is part of the seerah of the Prophet(saws), and his two daughters were married to relations, hence no Muslim can mock such a union.
The clan narration is well known and considered sahih;
“You all have become thin and weak, so marry outside the family.” [Iraqi, Takhrij Ahadith al-Ihya’]
The Hanbali school of thought has an opinion on this;
it is recommended for a man to marry outside the family, as doing so normally leads to healthier and stronger offspring. [Mawsu`a Kuwaitiyya]
But other schools do not.
As for the Hadith (marry far and wide, new tribes), the major Hadith masters have said the chain of narration for them is weak, but it wasn't an argument not to marry your cousin, as I clarified earlier in my post.
Here's a good article on this, backed by academic references.
https://themuslimtimes.info/2015/11/12/cousin-marriage-why-why-not-dr-lutf-ur-rehman/
I think this sums it up for me;
Although isolated cousin marriages may pose little risk, repeated consanguineous marriages within a group are more problematic.