We wait until the last jumping lesson of the day has been completed before setting the course and we do not allow anyone else to jump after the course has been set.
We store all jump decorations (flowers and trees) and they are only brought out for shows...so while our barn members may have seen the actual pieces of jumps, they don't regularly jump the decorations. When we have a horse that needs to be schooled over flowers, we usually use the random one-offs we have lying around.
I have had people walk their horses in the ring prior to the show starting. I don't allow it so I've asked them to leave. Both boarders and attendees do this. I have two schooling "breaks" that allows people to get into the show ring and hack prior to doing classes, and people can school in any class they don't compete in, so I don't think anyone needs to be in the ring prior to the shows start.
My trainer designs the courses, with minimal input from me. I might suggest placement of a single fence or suggest lines that could be ridden, but I'm mostly just physical labor when it comes to setting the course...so I wouldn't feel bad about showing. These days I don't show as I just don't have the time with everything else I do.
I don't know what "preferencing the host" means, so I can comment.
To sum up, I do think it's a bit of an ethical line...BUT not one I'm going to worry about as a rider. If a show host feels the need to give themselves or their students advantage by doing things I would consider unethical, well, that's kind of sad.