I think your confusion stems from the word "fashion", which we generally think of in terms of clothing. "Fashion" is one of those English words that has various meanings depending on the context in which it is used. In this case, the word is synonymous with "way" and "manner". Thus, it makes sense to use the following phrase (the phrase you used) when describing something which allows deploying apps in a very easy way/manner:
"It helps you to deploy apps in a very simple fashion"...which essentially has the same meaning as "It helps you to deploy apps in a very simple manner".
In answer to your second question, you could say "in the simplest possible fashion" if you are implying that the product has competition and is arguably the best out of all the others in the market. So for example:
Products A, B, and C (your product) allow apps to be deployed. But your product performs this function in the simplest possible fashion".
EDIT: however, using the phrase "simplest possible fashion" doesn't necessarily indicate that there are multiple products that do the same thing. Instead, it could refer to the fact that there are different methods available for deploying apps. In that case, "simplest fashion" would imply that your method is the easiest to use.
I hope that's helpful.