Can I coat a very soft cake with royal icing before I cover with fondant?
I don’t know that I’d suggest it
There are a few variations on royal icing, and many of them set quite hard.  This would be a problem for fondant coating, as you want the buttercream layer to act as a mastic to adhere the fondant to the cake
If the cake is especially soft, you’re not going to be able to use smoothing tools without crushing the cake
You might be able to use royal icing on the top of the cake if you use a ‘flood fill’ technique. I’d actually do it in two layers:  pipe a ring of stiff icing around the edge of the cake, then flood with royal icing and let it firm up. When you’re ready to apply the fondant, flood again and then set you fondant on the cake
By doing this, I hope that the first fondant layer will act to distribute force to a wider area, and the second wet layer acts as the mastic and deforms if you press too hard
To deal with the possibility of this being too sweet, you can use lemon juice in your royal icing to counter some of the sweetness