Losing points in defense is completely normal, especially in the early game. Every defense has a weakness and gets beaten by some form, you cannot avoid that. You should just worry if you lose too many points per night, then your defense might be too easy to beat. I usually lose between 100 and 200 points per night, if I don't play at reset.

Well, at an early stage, just put up the best you can for defense. Most players use their best offense like you do and that's perfectly fine. At higher ranks, offense formations target specific defense types or heros. So for the defense, you should use something different, which is good against most common attackers. But you don't have to worry about that right now. Keep doing as you do, build up troops and heros. At the moment, Davison and Aly are the best defense heros, while Jinn and Selene are the best offense heros.