I am not an expert or seller, just a pearl lover, but I would not worry about having the pearls match the dress. I have heard those who were fair-skinned say that they felt like the rose overtones made the pearls not stand out enough from their skin color and wished they would have stuck with silver overtones, I believe it looks brighter against fair skin. The ivory is not as popular, I don't think. I hear it looks better with darker skin. So, my vote would be white pearls with silver overtones.
8.0-8.5 is a very nice, large size. I think that's a wonderful choice.
Hanadama is of course certified and a sure bet as far as quality. However, you pay quite a lot more for the certified pearls. I would go with a trusted company and trust their AAA grade to be the equivalent of hanadama, without the certification and its added cost. Just because they are not certified "hanadama" does not mean they are not as good. The certification is expensive to get, so they can offer a better price without it.
I would get the earrings at the same time and ask them to match them to the strand, both in overtones and the metal color. If they're studs, the metal doesn't really show anyway but I think it just makes a nicer looking gift when the metal colors match. Also, personally I prefer the earrings a bit larger than the strand, so they stand out a bit rather than just looking like another pearl from the strand. As someone else mentioned, I would trust any of the regular sellers on here. If they are scammers or shoddy, they are quickly run off, haha (but really, they are). I say PP because they are so large and likely to have a large selection. If you use the chat function on the site, they will email you a few pics to choose from and you can ask your questions. (I'd use the chat function rather than emailing because I have found it's quicker response).
Let's see...
Hanadama- $2,924
http://www.pearlparadise.com/18inch-8085-mm-White-Hanadama-Akoya-Pearl-Necklace-744.aspx
AAA- $2,195
http://www.pearlparadise.com/8085-mm-White-Akoya-Pearl-Necklace99.aspx
I'm sure that hanadama certificate does carry additional bragging rights as well as the extra assurance of quality, so your finances and priorities for this gift figure in as well. But that is quite a chunk of change. Personally, I would get the AAA quality from a reputable dealer and keep the $800 difference (plus earrings, of course).
Again, I'm not an expert so that's just what I've picked up from hanging around here for awhile. I have a pair of hanadama earrings from PP, which are in my in my album on my profile page. They are gorgeous, but if I had known a bit more about pearls at the time, I would have gotten the AAA and saved some money. Maybe others will join in here. Good luck and let us know what you get!