There are some proportions on the bag, but they are usually for mixing the whole bag and are by weight for the powder and volume for the water.

Just add it a little at a time until you get the consitancy you want.

"2. How long do I have to wait to apply another coat of plaster using the 90 mix;"

Until the previous layer has set.
It does not have to be dry, just set.

"3. Should I spray the material I am going to put durabond on ie drywall?"

Ordinary drywall is fine without any dampening.

Make sure you apply Durabond neatly.
It is VERY hard and nearly impossible to sand.
Easysand (as the name implies) can be sanded without a huge effort.

Mix the water in, then add some more.
Mix again and repeat till you get the consistency you want.

After you do it a few times you will know how much water to add to get most of it the first time.

You also need to make sure you don't overflow the pan adding the water.

It is hard to mix very small amounts, or very large amounts in a mud pan.

Just remember you cannot change the setting time by adding more water.
Once the material starts to set it is not useful for very much longer.

A few or a couple hundred, depends( smiley face inserted here)

The 20 minute setting is much harder to deal with.