Rustic wedding cake with royal icing

Urgent help please! I have a rustic tiered wedding cake next week for an outdoor wedding where it’s likely to be either hot or raining. Rather than risk buttercream in those conditions, I am thinking of royal icing. My experience with royal icing as a cake covering isn’t wide so I have some questions I hope someone can answer.
I understand I can add corn syrup to the RI so that it stays a little softer, rather than turning rock hard – what quantity of corn syrup would I use?
The cakes are all mudcakes. From books I’ve referred to, it looks like I would have to cover the cakes with ganache and fondant first, and then add a single thin application of RI. I’m keen to find out what other people have done.
If I go the ganache and fondant route, I was thinking to deliver the cakes to the venue early (it’s 2 hours away) and apply the RI there. Is this wise – if the humidty is high, would it affect wet RI or will it dry quickly?
I’m planning to do a dummy run later this weekend – hope you can help me! Thank you!