2. On Declan being able to get energy from heat.
I think there are two possible answers, the first is a writing point and the second is a physics point
One, that it would make him too overpowered for the story or at least this point in the story
Who knows what sort of power he may be able to acquire in later books
Two, from a physics perspective, there is a vast difference between being able to impact something or initiating a process (this would be have magic impact it)
and a process being spontaneous or have free energy that can be taken (something reminiscent of Gibbs Free Energy).
So there is a question about the in world method of the Irwin ability to pull power externally for what it can allow.
Then there is the question about if Declan was able to attempt to pull the energy, how efficiently would he be able to do it.
If he is able to pull energy perfectly efficiently (0% loss due to conversion) than you could make all sorts of thermodynamics arguments that he can ignore entropy.
This means that there would be even more powerful exploits than pulling power from any weather, to an extent where it would break the world scale.
For a more reasoned version of this see wikipedia on Maxwell’s Demon (as in a though experiment) and look at the criticism.
So I would assume that if he can access power from something (like thermal energy as part of his fire affinity) that he has some power loss in conversion.
This idea of loss in conversion also includes the possibility it costs power to pull power.
This then means that anything has to have a certain power density (or absolute power content depending on world rules) to gain anything in pulling power from it.
It is likely that some weather would be above this available power threshold and some would not.
Overall, this seems like an possible path to power but not an efficient one.
Declan has access to Fairie through Omega and his drones. The drones can funnel back power (this has been seen in earlier books).
I would set up a concentrating array (or several) that funneled that power into something else (another drone or entangled component) that Declan had on him at all times.
So something like a necklace or a staff (which could accommodate multiple redundant concentrating arrays).
This concentrating array has the story benefit of not requiring weather (for example it could still work in space)
Then there are the writing benefits of not requiring a discussion of Thermodynamics, not stretching the so far presented world rules, being a logical (for Omega to suggest) extension of something seen and used in earlier books, and being something that can be scaled to story need and upgrade later on by Omega developing more efficient drones and the like