I worked with butylene glycol and I haven’t noticed it decreasing viscosity. It’s good and not expensive humectant. It is a glycol and decreases water activity which an additional benefit. I also find it less tacky than glycerin, so it is my humectant of choice for many o/w formulas.
If you want something for decreasing viscosity (for formulas with polymeric emulsifiers) go for humectants that are high in electrolytes such as sodium PCA (it’s a viscosity killer). But my problem is usually how not to decrease viscosity.
Silica.. it’s quite multipurpose. You can use it in a formula for oily skin indeed. Have a look at clinique’s pep start moisturiser so see silica used for oil absorption. I mostly use it in foundations and concealers though.
I haven’t worked with dicaprylyl carbonate.