You have a 2008 model, and your're doing valve adjustments? If you're not writing about the vehicle in your profile, you should identify by model and mileage.
Unless you've been running crappy fuel, plugs should be good for 80,000 mi, but why be uncertain? Replace the plugs and find out.
The #1 and #4 cylinders of a transverse 4-banger may run warmer at lower speeds because of fan placement.
20-22 mpg isn't unusual for mixed driving conditions. If it happened to me for 100 mile or longer stretches of continuous highway driving, I'd double-check the tire inflation, the weight of the cargo including passengers. When I have a full load in the back of my E, or
anything on the roof, the mpg drops significantly.
Next I'd keep an eye on the speedometer to see if I were driving faster than 60 mph.

MPG drops exponentially with wind resistance as sped increases above 60 mph. My Element loves the open highway; if I don't use the cruise control and keep pressure off the accelerator, it's so smooth (and fun to drive) that I'm constantly finding myself driving faster than I should.
I'm pretty sure that the 685# cargo rating of the Element isn't entirely based on what the vehicle can safely carry, but is needed to meet the EPA mpg numbers. (I believe that the EPA highway rating is based on 55 mph

).
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Coils don't gradually deteriorate from worn plugs; they
fail- from serious overheating, which can be caused by running an engine with good plugs too hard, from plug electrodes melting shut(uncommon) or high voltage cables with failing insulation shorting the coil secondary to ground. A shorted secondary causes the thinner primary coil winding to melt. The Element doesn't have high voltage cables.
With a coil per cylinder in a 4 cylinder engine, the duty cycle on the coils is a lot lower than that of a typical engine. If a plug were badly worn, its larger gap would lead to carbon buildup, unburnt fuel in the exhaust and misfiring, so the check engine alert would probably come on long before the coil could be damaged. If a coil were to go bad, you'd have symptoms like sluggish acceleration, a rough idle, find it difficult to reach and maintain highway speed.