Can epoxy fluidity be changed, before it is cured?
Some epoxies are more fluid at higher temperatures, at least at first.  The higher temperature also causes faster cooling.  I've used this to good effect with epoxy adhesives. A slow-setting (several hours to give a useful bond) product warmed up to about 40&deg;C gives a very good bond as it wets the surfaces well; it then cures fairly quickly, though not as quickly as the equivalent fast product.   Conversely chilling should both thicken and slow down the reaction.  Using a fairly liquid version and chilling for the thicker step might be worth a try, however it will still slump unless restrained by a mould.

Additives are likely to be problematic for thinning: they'll be liquids that are likely to want to evaporate, so adding too much will lead to significant shrinkage; they'll also affect the cure.  Water is a bad idea - it will lead to a cloudy and weak result, but small amounts of acetone or denatured alcohol can apparently be used if you want a very thin layer.  Mixing isn't easy, which means it's likely to introduce bubbles (as it did when I tried using alcohol-soluble dies.  So this probably isn't an option for forming or casting, but may help your topcoat.

Because formulations are proprietary, you'll have to do some experimenting on a smaller scale, and hope that translates well to a large scale. Other people may have used specific 
products that you can easily buy, and their results might help you get started.