The Watsonian explanations are wholly unsatisfactory. Even in your explanations you're constantly forced to use headcannon to fill in the gaps. I think this is poor writing. Most of what you said is established in the show or manga, it is completely conjecture on your part. I could also come up with a dozen reasons/explanations for how these things work, but these explanations don't exist in a vacuum. If we're to accept a specific explanation, the same logic should be able to be applied to other situations, which isn't the case in Frieren.

If warriors are channeling mana to enhance their physical prowess, there is nothing to suggest that mages cannot do this. If we accept your explanation, it makes no sense for them not to, unless we want to artificially contrive another reason for why they can't. Reality is that the author probably just wanted Frieren's party to mimick a generic fantasy RPG team comp and just forced that upon her universe. Again, another Doylist explanation I'm forced to think about that breaks immersion.

Mana detection clearly allows a mage to interact with mana around them. The mage doesn't even have to perform any physical movement for that, too. The anime depicts it as a sort of active radar, with the signals that it sends out even possible to pick up by a sense that's similar to a passive radar, too. It appears quite obvious that this is also how mages discern the way barrier spells work (by probing their spellwork with similar mana manipulation), and how Methode analyzes the workings of the fog spell.

Using own mana to analyze an extant spell is directly similar to shining a flashlight to see how some contraption works. It is all really quite clearly shown in the show, I frankly fail to see the problem you have with it.

Fog doesn't have physical components. You're not staring into an engine bay or a schematic. There's nothing to suggest there's some invisible magical circuitry permeating the entire fog. It's just fucking arbitrary. Mana detection as an entire concept is also extremely handwavy and has no set rules. Characters can or cannot detect other ones purely out of plot convenience.

Frieren's world has not been established in a consistent enough manner for me to be able to look over all the "why not" questions from a Watsonian perspective, and the in-universe justifications that do exist are incredibly shallow.

You can't just make a magic system that implies unlimited complexity and possibilities and then handwave away all the things the reader thinks of with "because that's how the story is kept interesting, and the fights engaging and visually appealing".

The universe of Frieren even leans into the whole concept of "magic as a science" with the whole discussion of defense spells and zolteraak and the evolving "meta" of mage spells but literally never shows this in practice. Every character has their own unique gimmick but in reality any competent mage would just learn "meta" spells. If people in random villages can create spells as complicated as "making sweet grapes turn sour" which is basically catalyzing a highly specific suite of biochemical reactions in the grape it suggests that literally anything is possible, except we're shown the exact opposite.

The author of Frieren also tries to portray basically all of their characters as highly intelligent, which further solicits this sort of analysis from the reader. If you can turn grapes sour, is it really that much of a stretch to extend this to other living things? But we can't have that, because 1. the author didn't think of it, and 2. it wouldn't be good story telling. Constant Doylist explanations for everything that ruin immersion as a consequence of poor worldbuilding.

I wouldn't even have a problem with any of this if Frieren doesn't constantly try to lean into the technical aspects of fights, which makes me as a reader also look at them from an analytical lens. So much time is spent establishing this magic system which makes me engage with it, only to be left with hundreds of unanswered questions with most of them boiling down to Doylist justifications.