Cal,
You can sharpen them either way... single or double bevel but if they already have a partial double bevel grind, that would be the easieast way to finish them up. When single bevel sharpening, the "unbeveled" side of the blade must be perfectly flat. It can't have a partial grind, nor can it have a extra lamination like the No Mercy or Eclipse singles do.
I love Zwicky and Eclipse broadheads in thier original double bevel designs but, it's very difficult to take a broadhead that was designed to be a double bevel and transform it to a single simply by grinding just one side. I have no doubt that's what caused your difficulty in getting the No Mercys sharp Mo.Huntin...and you are correct, all Grizzlies are RH, and RH singles perform best with RW fletch.
You can tell a RH single from a LH single by laying the broadhead on a table with the tip pointed away from you. Postitioned like this, if the bevel is visible on the RH blade, it's a RH single. If the bevel is visible on the LH blade, then it's a LH single.
Ron