For answer to 1, kindly turn to earlier posts quoted on the topic.
Regarding point 2, kindly further enlighten us about the Case no. and contents of the SC decision.
Thanks.
While I look for posts on scientific studies. You can check out these two links for Point 2 -
1. http://articles.timesofindia.indiatimes.com/2010-05-15/india/28311339_1_gotra-khap-marriage. This was the 1945 judgement. Though it spoke about Brahmin gotras, it does allow same-gotra marriages.
2. http://www.thehindu.com/news/cities/Delhi/article472462.ece
In his petitioner, Mr. Kadyan submitted that marriage within the same gotra was against Hindu traditions. He urged the Court to direct the Union Government to appoint a commission to suggest desired amendments to the Act.However, when the Bench asked him to cite any Hindu scripture which bans marriage within the same gotra, the petitioner failed to quote any. Observing that the petitioner was wasting the precious time of the Court, the Bench said when he could not substantiate his submission he should not move a court.