Additionally, if you dissolve the salt in the boiling water and the icing is still gritty then no matter what  icing recipe you use - it will be gritty. In addition to beet sugar sometimes causing this problem, some of the cheaper powdered sugars just are not fine enough. It isn't noticeable when people use heated type icings but it is in regular buttercreams.

Here in Canada, pure cane sugar is labelled as such. If it isn't, then it is beet sugar. Not sure of how it is labelled in the U. S. but I know you need a 10x sugar which means it went through a very fine sieve or mesh and therefore mixes in better. (It doesn't mean it was sifted ten times). The number refers to the size of the mesh. If your sugar is xxxx, it isn't fine enough.

Forgot to add this, another reason I wouldn't add the meringue powder to this recipe is that it may also make it gritty. Glad your ruffle piping worked this time. Otherwise I was going to suggest adding a teaspoon or two of clear piping gel per cup of buttercream but only for piping the ruffle because it may affect the taste a bit. This wouldn't matter for the small amount of icing in the ruffle but I wouldn't want it in all of the icing.