Okay, I am probably in the minority here, but I think it would be fine if you used [name]Everett[/name]. It is not like it is the same name as one of them. It just combines the two names. Personally, I do think the Rett nn helps as long as [name]Garrett[/name] does not go by the same nn. Plus, your child will have a different middle name. [name]Do[/name] they all have the same last name?
I think I would hesitate more if the children belong to one of your siblings. Not sure why as I know some cousins are close enough to be like sisters and brothers, but for me it is just something about letting my own sibling have the name they picked out without any sort of overlap.
That being said, I know two different families who have used the exact same name (first and middle) for cousins, second cousins etc. I think that is a bit odd as there are so many wonderful names out there. However, it was a family name thing.
In your case, though, it would be a different name.
To sum this up as I know I am rambling, I say go for [name]Everett[/name] if it is absolutely the favorite for you and your husband. If not, [name]Charles[/name] is an awesome name and you would not have any overlap there.
I just hate it for you that you have two names, [name]Rowan[/name] and [name]Everett[/name], that you even have to think twice about! The good news is that I have seen some of your other choices and they are great names as well.
I cannot wait to hear what you decide and whether or not you are having a girl, boy or twins! Congrats again!