Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure.
in combat(melee) does a leader add is leader bonus to himself
No. This is covered explicitly in rules 22.214.171.124 and 126.96.36.199 (especially the gray box).
and also every over unit he commands in melee?
Yes. This is covered explicitly in rule 188.8.131.52.
also does a Leader add is leader bonus to his own Morale check and Defensive fire check
No. (as per my first answer, above)
How do weapons break?
1. When a "Jammed" die trigger comes up when they are firing (see rules 184.108.40.206 and O220.127.116.11 and the Player Aid Sheet).
2. If hit by the Malfunction Event.
3. If you roll doubles when using the Sustained Fire Action.