Shawna, I am sorry that this is happening to you.
I think you did the absolute right thing taking your furkids to the vet and calling the police. Obviously this woman has no concept of right and wrong and is trying to profit from HER mistake, which is SO uncool. Also, make sure you take pictures of all of their wounds (including pulling back the fur to show them), and hold on to that vet receipt. That way you have concrete evidence if anything happens.
I just double checked, and Lowell does have a leash law, so there is no question that that mojo was illegally off leash. And if you can get your neighbors to say officially that they have seen the mojo be aggressive in the past, it makes the owner even more negligent.
As far as your homeowners association goes, I would not do anything until they try and do anything. Also, what is the exact wording on the agreement? It makes a difference. And what is the exact wording on your deed? I doubt that your backyard is communal property - it may be a shared space, but I suspect that you officially own it, which adds tresspassing to the list of issues.