Flavor – How to offset sweetness in a sauce

Nuts are much tastier when they are roasted. Dehydrating may be ok but it will not do the same thing for their flavor or texture.

Nuts have a lot of oil but there is not enough on their exterior for powders to stick to. Of course some of the powder would stick anyway- and with parmesan even more would when the cheese melted. If you are ok with a milder flavor and a lot of powder left in the bowl when you eat them then you could try to use less or no oil.

It is possible that you could make it work with some other liquid other than oil but oil tastes good and won't dry out. Additionally for spicy peanuts, capsaicin is soluble in fat so the oil will actually be intensified.

I don't know about an easier/less messy way because I don't consider this to be particularly difficult or messy. Just toss nuts with oil, then powder, spread on a pan and bake. Doesn't get much simpler than that.

Another alternative that has become popular is kettle roasted nuts. The nuts are cooked in a large kettle with an agitator to keep them from settling and burning. Usually they also candied. The process is much stickier and flavorings adhere much better. At home, oil popcorn poppers with agitators are available that can beautifully make kettle roasted nuts. Manual versions are sturdy and inexpensive.

You've got to understand what the purpose of the ingredients is. In a crumble or streusel topping (my go-to in the category is an apple crisp), the (gluten/wheat) flour, butter, and sugar are actually all binders/structural components. Taste/flavor is part of it, but as in your experiments in the other answer, you really can't vary one element wildly (separate from the others) without significantly changing the results.

With that in mind, the butter in a crumble/streusel topping will affect the tenderness of the crumb (similar to cake/pastry). Tenderness will also be affected by whether you used cold, melted, or softened butter (or a non-butter shortening, like margarine or lard/Crisco). The tenderness goes to mouth-feel, which will affect how/when your tongue picks up on the diff flavors.

Similarly, the temp/state of the butter/shortening affects how & how much the sugar will dissolve/interact with it. Depending on what type of sugar you're using (granulated/white, dark/light brown, raw/demerara, liquid sweeteners like honey, corn syrup, molasses, or agave), cold/melted/soft butter will just have a different holding capacity for sugar, which cooks can take advantage of to "super-sweeten" or "under-sweeten" given fundamentally the same amount of sugar.

And of course in a crumble, your perception of the sweetness of the finished topping will be impacted by the filling, since there's (delicious!) flavor interaction during cooking.

So it comes down to ingredient preparation & choice and cooking techniques - not just raw proportions. Cooking is a sort of applied chemistry - sometimes how you put things together is as/more important than what you put together.