So...he did no actual diagnostics? That is disappointing and must be frustrating. If there is an infection it will need more then what the fogger and diluted F10 is going to provide. If there is no infection, that is just additional stress she doesn't need.
With the severity of this and having popped up right after she slammed her head, I figured he would want to do x-rays to rule out breaks. And all it would have taken is a needle and syringe to see if any liquid could be aspirated. Which in turn (even if hardly anything came out) could have been looked at under the microscope. Sometimes you can see cells that are indicative of cancer. Or other issues.
If this is pus, there will be nothing your vet can "drain" per say, it will need to be surgically removed.
I'm not fussing at you, trust me. I'm just puzzled the vet would send you home without the most basic of diagnostic tests, and therefor missing the chance for some basic treatments depending on the issue.
Not sure I'd be happy with that.
But all that matters is that you are.
Still wishing all the best for the poor, poor girl. My heart goes out to her, that just looks so painful. She has the same name as I do, Lucy..