WashU will not consider grandparents a legacy. They only consider parents. However, it will demonstrate that you have some loyalty and familiarity with their campus and more reason why you want to be on their campus.
Although WashU is no longer considering demonstrated interest, they will still give preference to ED applicants. The demonstrated interest will only pertain to RD applicants.
If WashU is your first choice and it’s where you want to be, I would go for ED. BU has ED2 so that could be a strategy as well. I presume ED1 has a higher acceptance rate than ED2 for both schools.
I wouldn’t worry about your friend applying. If you both get in, that would be great! It’s out of your hands if one gets in or one doesn’t and if you have a great friendship, it will still hold. Your applications will have no bearing on the the admissions office. If you are both qualified and they want you, they will take you both.
In terms of your stats, you are right where you need to be. It’s a matter of fit and which school you would prefer.
Good luck to you!