Assault & Battery: First Time Battery Charge

My question involves criminal law for the state of: California

Background:

I recently had an altercation with an Ex-girlfriend which just so happens to be the mother of my child.

At the time of the incident the sister of the Ex was living in my home. During the altercation there was no physical contact between the Ex and I; however, the sister stated to the responding officers that "I saw him hit her". This as you can imagine ended turned the call into a He said- She said type of disturbance. To the best of my knowledge she did not have and marks, scrapes or bruises on her. I was moderately intoxicated and I ended up spending the night in jail being charged with Misdemenor battery. According to the officer that filed the report the Ex was not pressing charges.

We now fast forward to present day and the Ex is threatenting to press charges against me because we are having difficulty coming to an agreement on child custody. I personally believe she is trying to now file charges to use against me in a custody battle. Although I was moderately intoxicated at the time; I have NO criminal record.

The question/s I have:

If she did end up having bruises/marks (from her own doing) wouldnt I have been charged with felony battery?

Can she press charges after the fact?

Can the DA build a case solely on a witness' statement?

Thank you :wallbang: