When I was making practice buttercream for the wilton decorating classes I did a few years ago they made us make stiff, medium and thin consistencies for icing.
For roses, we used the stiff consistency. Sometimes I found that I had to add more liquid than what was called for to make it flow better. Even though you doubled the water and corn syrup, I don’t think it would hurt to add more until the consistency flowed comfortable for you. We also added meringue powder so that the buttercream would crust over. And it definitely needs to be a room temp to flow properly.
This icing that we made contained icing sugar, shortening, butter, vanilla and milk. Which we also had to use to fill and frost the cakes. This was before I discovered Rose’s buttercream recipes. I could not imagine covering a cake with wilton’s icing now.