How long do you want to keep the decorated dummies?
One of my accounts used to cover a styro dummy with regular buttercream for the client tastings they had once a month. They would keep for a day or two then wash them off. Since they used real buttercream, they kept it refrigerated so the butter hardens.
Depending on how long you want to keep them, you could use royal icing as pretend buttercream; knowing that it will eventually dry. You could even sand them smooth once the royal icing dried - this is how the cakes in the old Wilton books looked so perfect!! The royal icing will eventually attract bugs. It can be colored with regular gel/liquid/paste colors and your usual tips and bags.
You can also use spackle, but this will eventually crack and discolor. You should be careful not to use any of your tools for regular cake decorating with this stuff. There is a specific fake buttercream made for this, I know of one called PermaIce - it requires separate tools, and special color (you can’t use regular colors with it).
Regardless of the method you use to cover, do as Jen does and run a long bamboo skewer or dowel to anchor them all together. Make them in advance so they dry - at least enough to prevent any marring when you transport them.
If you end up using fondant, you should stick the flowers in while the fondant is still soft; once it hardens, it’s like a rock and nothing will penetrate it. You’d have at least a day or two with the fondant still soft depending on what brand of fondant you were using.