What would you use to attach heavy decorations to the side of a fondant cake? I usually use tylose and water but wondered if melted choc or royal icing would be stronger. 

I always use melted white chocolate, especially with heavy item as you can set the chocolate quickly with an ice spray. 

I also thought the melted chocolate would be good 

I don’t have any of the ice spray and can’t get it in time. Will it work without? How long does chocolate take to set?

Yes, it will work without the spray. You just have to hold it until it starts to set, setting time will depend on how warm it is. 

It shouldnt take very long — at most 30 to 60 seconds  but depends on how cool the workroom & chocolate is and exactly how heavy the item 

I also use melted chocolate for heavy decorations, otherwise, I use Tylose glue.