you can put it in any time you like. before . during or after.
^^^ completely wrong. You must add salt after the specified time that the plaster manufacturer says.
That depends on the size of the pool and how fast the water is running that is filling it.
Add salt
This needs to be tested with a pool salt testing device or get the water tested at the pool shop.
Yes salt water pools are chlorinated pool any way.
If you have a salt-water pool and you have an indicator that says add salt it means that the amount of salt in the pool is not enough for the chlorinator to manufacture chlorine. Take a sample of water to your pool shop and they will help you with what is required. Or get a couple of bags of salt and add salt until the salt warning stops. The latter is a bit of a dodgy way of going about it.
The usual reason for salt in a pool is for chlorination. Epsom salt won't work with salt/chlorine generators. BUT, you can add epsom salt to your water.
Add acid
No you can add as many as you wish
Yes there is no need to drain a chlorinated pool to change it to salt water pool all you have to do is add the salt install the new equipment and run it as usual.
Yes as a mater of fact that is what salt water pool normally is. With a salt water pool there is a electronic salt water chlorinator installed that uses the salt in the water to create chlorine. However if you don't have a salt water chlorinator and prefer the feeling of a salt water pool then there is no reason not to add salt to the pool as well as keeping up the chlorine yourself.
Yes, you leave the filter on in order for you to put salt in it
It depends how many gallons your pool is. Once you buy your chlorinating system, the install manual will tell you how much salt to add based on how many gallons in the pool. I have an Autopilot Digital and it tells me when it needs salt. I still check it with test strips to be safe.