If your H1B is valid and you meet the requirements (you are working), then you will keep your H1B status until your F-1 is approved. Then your F-1 status will start from the date immigration gives you. It should be the date school starts (the date on your I-20 from the school) but they can make it different. When you apply you can indicate that you are working and wish to continue work up until school starts. But it should be OK, usually for university they make the COS date the date on the I-20.
1) Status changes on the date that immigration puts on your approval. Should be same as I-20 date. In other words, change should be from when school starts
2) Up to you. The honest thing is to tell the employer when you apply for COS, but I think it depends on how you think they will react. If you think they are understanding you can tell them when you apply.
3) Depends. If you employer will keep you and H1B is still valid, should be OK. But I really really doubt this will happen. Most people want to chage F-1 to H1B! Changing from H1b to F-1 is kind of going down a step. i think approval should be 99.9%
You know you can study on H1B as long as you maintain your status through employment. Why give up the H1B and job, especially in this economy? I dont know your case, but just generally, a lot of people on F-1 would kill for an H1B.