Re: marrying first cousins?
Yeh. If anyone else read my link they would have read that the increased risk posed by cousin marriages for stuff like downs is the same as pregnancy at an increased age.
Stuff like heart disease and diabetes has more to do with ethnicity and then environmental factors i.e. your diet and lifestyle. All desi people have a higher risk of type ii diabetes.
As for rare genetic disorders, if people are marrying within family generation after generation, these will present and then it will be clear the practice should stop within that family.