trig42: It will be OK, getting a diamond ring from a manufacturing jeweller will cost a lot more than $1500. If you had paid full price, you would be right to feel ripped off, but if an independant valuer has valued it at over $2000, you got a good deal.
Congratulations on the upcoming event, don't let this taint it - it will be great.
I agree with the above. It doesnt sound like you got riped off. I understand the frustration of their grading system not meeting the independent grading. I think you have a fair case for some kind of compensation. How about a store credit (you only wanted $100) for you to use on the wedding rings??
If you had a small budget then you have to give a little in some or all of the gradings. Clarity to me is not the most important issue (as long as it is above the naked eye range), it would be the cut. The better the cut the more it sparkles. Also grading a diamond under .5 of a carat is not recommended (due to the diamond size) so if it is less than .5 you will likely have big variances in the grade. If you gave it to 5 different valuers at that size you could get quite different valuations.