I think glaze has corn syrup in it which makes it shiny and doesn't dry completely rock hard..Toba Garret has a wonderful glaze recipe...Thinned RI is just meringue and water/cream of tatrtar for some recipes and icing sugar.Antonia74 has a awesome RI recipe for cookies.
A few months ago I did an experiment where I compared the two types of icing. Maybe my experience will help answer some of your questions. I posted my results here.
Bonniebakes icing experiment RI vs glace
http://cakecentral.com/cake-decorating-ftopict-684108-.html
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