"Combust" as a technical term is rapid burning of fuel in the presence of oxygen so technically speaking, no, "air" on its own generally can't combust through normal means because there isn't a high enough fuel/oxy ratio...but this is magic. You can come up different explanations for how it's done at a molecular level but at the end of the day, using magic to bend rules like localized entropy and enthalpy, it's not hard to affect "combustion" of the air (which isn't only oxygen, CO2, and nitrogen...) to create a ball of actual fire, whereas unless you're in a REALLY dusty place, you're not going to pull earth from the air. And no, fire magic can't be used only do that, but it's the most common use.

Who said it was easy? Magic works totally differently for everyone based on their teaching, experiences, and belief systems. Not all elements come to all wizards equally (obviously; look at Harry vs Hernandez). Everyone has their preference. It's arguable that lightning is a far better element to attack someone is than fire due to fallout and property damage issues (when "purification" isn't an issue), but lightning doesn't come as easily to some wizards as it does to others. Harry has said he pretty much is incapable of water evocation. I get the impression--based on Hernandez's example, what we know about elements and personality types, and the nature of water in a metaphysical sense--that you've got to have a certain "go with the flow" mindset to use it well--something Wardens tend to be short on, I'm betting.

I don't think it's worth overthinking or penalizing your players based on what element they choose. 

Why should spirit be any better than water, or earth?  What is spirit magic made from?  How is it any more likely to be effective?  It's not even a 'thing'.

I think the elements you need are around you.

Leave it to the realm of compels and shifts of power.  If something requires too much power or volume, then compel.

If you are trying to ground out a spell or a circle, that is a counter-spell based on the power of the spell you are countering.  Let the player decide how it works.  Water erodes, Fire purifies, earth grounds it out, air spreads out the energies.

If you are trying to block magic, it's a block action.  The type of magic dictates the narrative.  If it's really dry and you don't think they should be able to pull water out of the air or summon it, use compels.

Compels should be used for the 'weakness' of any type of magic.  Other than that, it should only be limited by your player's imagination.

Same with their strengths.  If there's lots of water - whether it's in the Nevernever or no, you can use a compel to have it ground out magic, or if the players want to manipulate it, offer a compel to say that body of water acts as a 2-shift threshold.  Now all their magic is harder to use.

Fire goes out of control

Earth is inert.  Once you make a big pile of dirt or throw rocks around, it can create a scene aspect/obstacle.

So, yeah.  I'm all for letting people whip up whatever element they want until it bites them in the ass...or until they can't.  All compels.