if they want to
Just like any other criminal charge. Probable cause to believe that the offense occurred.
It depends on the state. Some do not allow "assault on assault" charges.
Most likely no. California recognizes out-of-state domestic partnerships that are substantially similar to marriage. A Maryland state registered domestic partnership offers only limited rights and is not substantially similar to marriage. Therefore, California will likely not recognize Maryland state registered domestic partnerships.
Domestic assault in Tennessee is an assault against a victim. The state prohibits the abuse of current or former family members or romantic partners.
If you were charged with a felony assault and the charges were dropped, you do not have a criminal record. The charges can be picked up by the state at a later date.
One way to file assault charges in the state of Virginia is to have police officer come to your home. A person can also visit the local courthouse and press charges there.
You can file assault charges in any state.
Will depend on convictions, not charges. If the assault was a conviction of a charge of violence against a domestic partner, you may not buy or possess a firearm anywhere in the US. Period. This is covered under Federal law, not state.
You will get off with a misdemeanor penalty if you have money, no prior convictions and don't have lots of other pending charges. The victim has not say in the case and usually ends up broke on state aid and a victim again. If you are a criminal you have rights if your a victim you just have injury. Sorry victims.
Consult with a lawyer familiar with Maryland state firearms laws.
If you assault a non-family member then domestic violence isn't part of the equation. Each state has different terminology on "assault" and I am sure that some lawyerly individual will add to this. Added: Any offense involving a charge of domestic violence or domestic assault automatically "enhances" the assault charge. Conviction of any offense enhanced by a domestic violence charge will prevent you from ever owning or possessing a firearm. (Lautenberg Amendment).
No. Assault in almost all forms is illegal.