Number one most important thing is to give her ALL the medication even if it has already "cleared up". Ending antibiotics prior to the full course encourages the formation of antibiotic resistant bugs and the ear infection will come back, worse. You noticed the problem the first time, so watch for a return of the same type of symptoms to make sure it is gone. Most of teh time, if the medication is given appropriately and completely, it will clear it up. You could always take her back just to be sure.
The second question is harder. There are tons of theories without a lot of proof as to things that make your child more or less likely to get ear infections. Breast fed babies definately have less ear infections, but at 5 months old if you haven't already that isn't something you can do anything about. Childrens ear tubes do not have enough tilt in them to facilitate proper drainage. That is why kids get more ear infections than adults. As they grow the tubes get more of an angle to them and it prevents the stasis of any fluids or debris that cause the infection. There is nothing you can do to change this, she just has to grow!
As to the third question...I have 2 boys, 3 and 6 and between the two have only had 3 ear infections in the last 6 years. My children never ran a fever, never got fussy, never even played with their ears. One was diagnosed because I took him in for his runny, stuffy nose (allergy issues)and the doc found it. One had a cough I took him in for and doc found it. The very first one I found while putting him in his carseat and saw nasty stuff draining out of his ear. I was very distressed that I was a terrible mom, my baby has an ear infection and I didn't even notice until it got that bad! I was reassured by the doc that frequently children have little to no symptoms and it wasn't surprising I hadn't picked up on it.