I organized a successful schooling jumper show for years. We stated that we followed USEF rules in the prize list so that if there was any confusion about eliminations or other matters we had a rule book to refer to. The fact my judge was also a USEF steward came in handy once or twice - schooling show or not people are competitive and we had great prizes. 
As everyone said, three classes are all you need per division. Optimum time instead of a speed class, power and speed for the second, and then a jump off class. I’d probably list it as a/b in the prize list and then decide day of whether people were jumping off directly or coming out. Some riders just want to get it done and others want to experience coming back. Only you know your local crowd.
In our area people do walk courses so we allowed time for that before each division. Some people would also do a quick walk while we switched the fence numbers. We did not run an open card. We usually had a lot of entries and it was just too much to keep track of. If you do, you will need to have different colored numbers for the fences so people will know which fence they are looking for. That’s another reason we just did one class at a time. But I was lucky to have 3 rings going.
We used to do two special money classes at the end of the day at higher heights. Usually a Mini Prix and a Gambler’s Choice. Honestly the numbers for the Gambler’s started falling off even with prize money. It’s a lot of work because you have to move the fences to make sure they can be jumped both directions. It’s a fun class if the interest is there and you have good riders who go in with a plan. If not - it’s bad. I wouldn’t recommend it at lower heights.
Running a show is a lot of work but it is great fun. The smiles on the faces of happy riders makes it all worth it!