Sorry to hear about your recent attack, its worrying for sure. Owls can be quite a formidable predator. I couldn't quite see from the video but I wouldn't rule it out.
I would report to the local authorities or knock on a few doors locally to find the owners. It may well be the dog is escaping unbeknown to the owners and it's likely very local.
Vegetables tend to work well; especially cabbage hung from the coop! Gives your birds plenty to do and keeps them quite entertained.
Other than that asparagus and green beans are particularly good.
For fruits, apples, mangos and anything easy to consume such as cucumber!
Would just like to second the point on the nest box being sufficiently large enough, 12x12 being the minimum.
It's also advised to have 1 nest box for every 3 hens, so you may want to consider 5 nest boxes over 3.
All the best.
Perhaps the safest and quickest way would be to contact a local pest infestation company. They should be able to give you some direction and advice at the very minimum.
Aside from this ensure you are not leaving any food out to come the end of the day, the feeders are not accessible to rodents...
Moderation is certainly the key if you are going to offer scraps. Around 10% of the total diet is a good rule of thumb to generally aim for.
But it does also depend on the type of scrap you are looking to offer and the season.