What are the benefits of whisking when making muffins

The difference was definitely not caused by the salted or unsalted butter. It is also very unlikely that the problem was the flour.

Some of the usual reasons for muffins to come out flat are:

• you forgot the baking powder (but then they are a complete failure, I guess you would have noticed if that had happened)
• you measured by volume instead of by weight. This is very imprecise and can easily result in too little baking powder for the amount of other ingredients. You should use a recipe given in grams and a kitchen scale, if you want consistent results.
• you let the prepared batter sit for too long before putting it into the oven, and the baking powder spent itself before the baking began
• you baked at a wrong temperature, so the batter hardened either too early or too late.
• you used ingredients at the wrong temperature (your eggs came from the fridge, your butter wasn't softened or you tried to soften it in a warm place and it melted, etc.) Always start with room temperature ingredients for baking.

You don't give us enough information to know which happened, but if you want to have good muffins every time, avoid these common mistakes and your muffins have a good chance of looking good in every batch.

Since it sounds like they are basically a dried out batter at this point, I would turn to implementations that rely on those kind of food-stuffs;

• With some further baking and possibly additional brown sugar and butter or apple-sauce, I would use them for a crumble topping. If they are coffee and cinnamon an apple-cardamom cobbler would benefit nicely.
• The crumbles could be incorporated into a biscuit batter recipe
• In all likelihood, any kind of recipe that relies on a granola could probably be referenced for further ideas.
• You could also use the crumble as a stand-in for oat based no-bakes. The no-bakes might feel mealy to the tooth, so not entirely eliminating the oat might work best.

If you wanted to repurpose, you might try thinking of it as a material to be suspended. With an ingredient like well-blended coconut butter (not to be confused with coconut oil or milk or cream) you could create a base with which to make a kind of post-raw bar cookie (basically press and chill the coconut butter in a pan on top of wax/parchment paper, pulse the muffin bits with some dates or brown rice syrup, and once the butter sets press the bits/dates mixture into the top and drizzle with some ganache or whatever).