Should one of my miscreant hounds ever require an attorney, you counselor, are it!
Said mastiffs were neither on the owner's property, or under the owner's control. Therefore counselor, the owners and not the dogs, were in violation of, I am reasonably sure, several local ordinances and state laws. Although since your assiduous research has turned up scant statutory evidence, the Wabash Valley in Indiana may well be a lawless frontier region.
Although it is certainly an intriguing legal concept, the owners have no right in law to permit their mastiffs to give a "righteous chewing" to any dog, stray or not. Of course a plea of "self-defense" might be entered. You have offered a "guilty with explanation," sort of plea here on behalf your canine clients.
Get back to the humans. The owners of the maligned mastiffs will no doubt bring suit, citing emotional distress, property damage, and irreparable psychological harm to the children who witnessed the horrible scene. They have a sort of half-fast case. Should be interesting. Of course, the stray will have to be subpoenaed as a witness.
Dogs that defend their property from a stray claiming control of said property are as justified as doing so as any Freeper who defends his or her property from any other criminal.
In my opinion, the Freepers would still be justified if, during a scuffle, the fight crossed off their property onto a vacant field.
The dogs were under control until they were provoked. There is no evidence to indicate that the owner was deliberately ignoring the dogs; with two young children, she may have had duties to them that were more important than seeing what her dogs were doing. No doubt, she had heard them growl and bark at that stray before, and she seems to have assumed that the matter could wait while she took care of other business.
Since there is no indication that either of the dogs had ever previously broken its chain, it stands to reason that the owner assumed (albeit falsely) that the chain was strong enough to keep the dog on the property.
While the dogs were, apparently, off the owner’s property when shot, their handling was a matter for animal control or for catch poles, if they truly had not been able to separate the dogs by any other method.
Fine the owner, if you must; shooting the dogs is overkill.