Rye is really simple - you will see you are overthinking this after your first application of good seed. You are not looking for year long results so simply broadcasting the seeds over and watering properly to keep the seeds moist and seven days later you will have a rye lawn. During this time of year in our area it does very very well especially once we are below the mid 80's.
Truthfully IMHO rather than trying to fight disease that may come along could prove more expensive than getting another bag of seed and repairing the areas as needed. Remember this lawn is ready in roughly 7 days.
My recommendations would be easy on the N since it is just for a 1 time event. This turf will grow like crazy during this time of year. You will get several inches of growth in a two week period if allowed to go that long. I would inform the owner that to keep it nice and tidy looking he will most likely need weekly mowing.
Last but not least - this turf stain much worse than most of our southern turfs. Clippings on any paved area will stain if stepped on prior to removing or if the mower tire runs over it before they are removed with the blower. This would be a big concern I think you should ensure the owner covers with the bride who I would presume would make a appearance in her white wedding gown - which of course cost a good amount of money - and I can almost assure you especially if the grass is freshly cut, it will stain that dress or any other material that is dragged across it.
10% of your customers are 90% of the problem.
You may think the grass is GREENER on the other side, but if you take the time to water your own grass it would be just as GREEN.