Water Leak from Neighbor AC unit

What is the name of your state? Virginia

Neighbor's condensation drainage pipe was misaligned causing water to seep into our adjoining wall and causing $3500 of water/mold property damage. Their insurance (Nationwide) rejected the claim citing our neighbor were not negliant because the pipe 'may' have been misalign due to longterm vibration and neighbors acted with due diligence once notified. With that there are not legally liable as well. My insurance rejected the claim because their was mold (they have a mold exclusion).

Tried to settle with my neighbor directly but that failed. I am now taking my neighbor to civil court for property damages substained.

A couple of questions here:
1) Is ignorance a good defence against liability? It is not disputed that the property damage source is my neighbor's faultly equipment but rather my neighbor was not originally aware of it?

2) Do I have to prove neighbor was neglient or do I can I simply state there are responsible for equipment on their premise and therefore at fault?

3) What is the definition of neglience? Is it ill-intent or unattentive?

Also, the damage was not visible until the exterior wall was removed and then it was obvious and substantial. The contractor doubted we could have this type of damage and my neighbor to have none. I am not aware any remedial action that neighbor performed to effect repairs on their side of the wall.

3) Can I legally request proof of inspection or repair?
4) If I can prove they did not effect repairs does that prove neglience?

Trying to figure out if I do have legitimate case or not. I kind of thought this fit the 'baseball through the window' analogy but then I can not figure out the other side arguement or what is the 'letter-of-the-law'?

Any knowledgable insight is appreciated,

Michael