How much molding paste can I mix with Gesso? Can I add water to the mixture to make it smoother?
The ratio of Gesso to Molding Paste can be what you desire. Try to create a thinner mix by adding more Gesso instead of water, then add water as a last resort. This will result in the strongest film.

Is there a certain amount of time I have to wait for the Molding Paste to fully cure before I can paint with oils over it?
Even though the acrylics touch dry very quickly, they don't really cure until several days (or more) after application. They FULLY cure in several weeks, but it is not important to wait for this curing, as there is very little water and additives left in the film which could impede adhesion. If there is too much water and additives left in the film when you apply the oil paints, they won't form as good of a bond to the acrylic surface.

I would like to know how to apply Molding Paste very smoothly. I need to have the most polished surface that can be.
My recommendation is to work on a panel with the Hard Molding Paste. The hard version has more leveling and will be easier to sand smooth than the regular version.

When I have made smooth panels, I used a silk- screening squeegee instead of a brush or spatula. If the panels are very large, I would try to make one out of wood or plexiglas and make sure the edge is extremely smooth.

You can also do this with the Regular Molding Paste, but it won't self level nearly as well as the Hard Molding Paste. However, if this is for stretched canvas, I would suggest still to use the squeegee, but try to do skim coats and scrape thin layers until you build up the desired thickness.

With either product, you want to let it dry thoroughly before sanding. Sanding between coats isn't necessary.