1. Females have shorter fins.. plakats are a bit harder to tell apart, but you won't likely find an unlabeled one in a LFS since they're a little pricey.
1) Females are as everyone has said. Actually you can buy a plakat at Petco, but they are bit more. They are $12.99 now but they are almost twice the size of normal male betta.
2) I really wouldn't buy the "betta water" I work at Petco and even we think that it is a ripe off, $4.99 for 2 quarts of water. It is really geared for people who have their bettas in a bowl, because it says that the water is oxygenated. Test your water out of the tap don't use the strips either. If it is that bad just use Prime and tap water.
3) I have some shrimp in my 3 gallon tank and so far so good but it is a gamble and the betta will probably win.
4) I do a 20% weekly
5) My tank is a constant 78
6) I feed mine 3-4 pellets once a day
7) I have live plants so I leave my lights about 8 hours
8) I don't see a need for an air pump if you have the filter that it comes with. I have the same tank but I upgraded the filter since I'm not a fan of the Whisper internal filter. Also if you use an air pump you will have faster evaporation.
9) The life span length is had to say since most bettas are adults when you get them at the store. I have one now that I have had for over 2 years now.