The character has control over the electroweak force and uses it to induce a directed, continuous combustible reaction in the air between oxygen, hydrogen and nitrogen which all started by a small electrical discharge
The full electroweak interaction is not required to control chemistry. All you need is the electromagnetic force. I'd suggest not complicating your life with the electroweak interaction in full unless you really have a good reason to.
I see what you're trying to do (nuclear physics), but you don't need it (IMO) and it's likely counter productive.
Combustion versus Splitting Atoms
Combustion is a very low energy chemical reaction compared to nuclear reactions.
Chemical bonds are many orders of magnitude lower in energy compared to nuclear bonds.
If your character can control nuclear interaction they absolutely do not need to worry about combustion to generate energy. They'd be, as it were, up to their necks in energy !
What we're talking about here is the difference between a tank of gasoline exploding and a nuclear warhead.
The problem your character will have is that splitting Oxygen into Hydrogen atoms with some "electroweak magic" involves enormous quantities of energy and these energies are beyond a useful human level of use. It would depend somewhat on the details of your magic system, but a lot of energy is in play - way too much IMO.
Thing is, when comes to direction, I can’t figure out a way that by using either force to create a concentration of the needed gases in a, let’s say, cone that is one meter in width and eighteen meters in length; unless the character causes the break down of oxygen atoms to hydrogen atoms and thus creating the sample mentioned above.
Hydrogen, Oxygen and Nitrogen are everywhere. They're easy to obtain, especially if your character has control over (just) the electromagnetic forces. They can break bonds and control electric fields and direct particles.
Molecular oxygen needs very little encouragement to bond to molecular hydrogen. Note that free atoms (non-molecular) are not the norm in nature.
There's simply no need to go (dangerously) nuclear to get these chemicals if your character controls electromagnetic forces in some magical way.
All your character needs to do is get water vapor into reasonable concentration in a given area and then magically split the atoms apart whereupon the now freed atoms would almost immediately start trying to bond once your character restores the fields to normal.
Moving water molecules can be done using electromagnetic fields as the different molecules have different properties. Your character can also modify these with suitably chosen "electromagnetic magic".
The energies involved are what I would term "human level" as they are at an everyday level humans are used to seeing and dealing with.
From a creative modelling point of view the maths here is simpler (not necessarily easy, but simpler than the nuclear option). Explaining this (and glossing over the details) is easier to write about and read about.
The nuclear option - splitting Oxygen
Splitting (by magic) stable Oxygen isotopes into Hydrogen has numerous problems.
As mentioned there's the enormous level of energy involved. These are bound states and the binding energy of these nuclei is such that you have to do the equivalent of pumping in vast amounts of energy to split Oxygen nuclei.
Note that once split it's a lot harder to stuck nuclei together (fusion) than it was to split them. So reversing the process is harder.
Assuming your magic takes care of that binding energy problem there is the trivial problem of lethal neutron radiation !
An Oxygen-16 nucleus has 8 protons and 8 neutrons. A normal hydrogen nucleus is just a proton. Lots of neutrons suddenly released.
To avoid this your magic has to allow the Oxygen nucleus to be split not into plain old Hydrogen, but into Deuterium - the isotope of Hydrogen with one proton and one neutron. The proper name for this isotope is Deuteron but let's keep it simple.
There will still be problems. Splitting the nucleus doesn't mean you get a conveniently harmless neutral gas. You've just created an extremely dangerous ball of plasma - charged deuterium nuclei and electrons which are not bound to each other (yet) in nice neutral atoms. Stopping an explosion is not going to be a problem - controlling one is ! These particles will try and bond with anything they can, including themselves and any container or other molecules in the air.
Side note : Deuteron is a stable nucleus and makes no practical difference to normal chemistry for Hydrogen.
So what you get with the nuclear option is a chaotic mess that involves huge energies and can't be easily reversed.
I really don't recommend this approach. The energies and states of matter involved are not at a human level and will be very complex. Writing and reading about these will be very complex and turn a work of fiction that should be focused on story-telling, narrative and character interactions into something more like an undergraduate text on particle physics. It's not a direction I recommend for that reason alone - you'll be explaining way more than the reader wants to know (or end up with a very small audience who'll get into it).