Today seems to be alternator warning light day… You might check some of the other alternator warning light questions as the alternator and its supporting circuitry only has a few variations.
You can do a quick check of the alternator if you have access to a DMM (digital multi-meter, an analog one would work fine for this too). With the engine running, alternator output should be well above the battery voltage. Expect the battery to be about 12.6 volts if it is fully charged, anything less that 12 volts is very low. If the alternator is working you should see something around 13.5 to 14.2 volts. You may need to rev the engine up a bit to know for sure, if you watch the voltage as you increase engine speed you should see it increase (if it was low).
The fuel consumption is a bit harder to offer good advice on. But a few ideas come to mind:
Make sure that you're taking the long view, instantaneous or short term fuel consumption isn't necessarily diagnostic. If it stays high, that is another mater.
Fuel consumption will go up if the engine is asked to do more work. It is conceivable that the load could be coming from the alternator, but you're looking at an almost 50% increase and that seems kind of high for an alternator load.
Restricting the available air might also reduce fuel economy. If blowing out the filter wasn't a reflex (and give how hard it is to get to filters these days, that seems a bit unlikely) then whomever did it probably thought the filter was dirty. So there might be an air restriction. A quick and dirty (pun intended) test for this would be to remove the air filter and drive a short distance. If the filter was the problem the fuel consumption should go back towards what you expect to see. Don't do much driving without the filter, just enough to see if you notice an immediate difference.