You can edit your prior post; you should do that because the link is confusing. Just replace the incorrect post with something like <error - see next post >
OP, I agree that it's unlikely a detector will help you solve this mystery because as you found there is a lot of metal in a home, and even the best detectors (which cost $$$) can't perfectly discriminate between metal types.
I suspect that a manual search by family/friends might be the best approach. I can't imagine that your attic is more than 1-2,000 square feet. I think a few people could manually search that in an afternoon.
Do you remember what the precious metals were stored in? (a box? what color?) 100 silver rounds and 2oz of gold should be about the size of a tissue box.
I'm also assuming your attic has the classic batt insulation; your search party could make one pass through the attic without removing it all and then if the treasure isn't found you could even remove the insulation one bay at a time and look.
Are there other items in the attic? If so, this is a good opportunity to clean up and throw out anything not needed.
And finally: are you 100% sure the coins are still in the attic? Even if you aren't 100% sure, the search is worth it. (Story #1: my son and I were asked to search for a wedding ring on a beach once, so we set up a search grid and spent about 90 minutes searching but didn't find it. The owner came down from the condo and said, "oops - the ring was up in the condo the whole time". My son and I still had fun together, and I got a great picture of our "grid" from up high.)
(Story #2: if by chance you never find the coins, I know the financial cost will be difficult to bear, but please know that someone in the future will find them and you will make them extremely happy. I happened to find silver coins hidden by my home's previous owner in our basement back in 2015; you can check it out in this link:
http://www.treasurenet.com/forums/coin-roll-hunting/466360-found-treasure-hidden-basement.html)
Good luck to you!