1. If a ball is at 1000 before the start of league what grit would it be at end(med-heavy oil)?

There's a lot of variables which would make it hard to give a exact answer.

Here's a video that will give you a idea.

2. Should you refresh the surface before the start of each league if the oil volume is the same or does a cleaning bring it back?

Just cleaning won't bring it fully back to the surface you started with. Refreshing is typically recommended after about every 9-10 games, but it really depends on what surface you have and the conditions you bowl on.

3. If I wanted to go from 1000 to 1500 should I hit it with 500 first?

Sanding with 500 grit first then a 1000 grit doesn't give you 1500 grit.

Unless you have a machine to measure, you will never know the finished grit. Focus on the reaction you get and adjust the grit based on what you are looking for.