Go back to the begining. Remember the places in the coop where the chicks can't fit in order to get out is not the purpose of filling those spots. Some fences are to keep things in. Some are to keep things out. You've got the "keep things out" situation here. That was you first mistake.
Did you look at your coop as predator proof? If not you have real issues to correct.
The 20 year old, wow, what a problem you have there. If he was lax enough to let his own dog get picked up by the pound, what else did he let go? He may be a good boy and never gives you a headache, but he's not very responsible. That will probably come with maturity
We all feel for you and the loss of the chickens. That is a terrible was to come home from a trip. All you have to do now is to fix the problems and get some more chicks. 10 days is to much time for me to be away from mine. I've canceled things the wife and I were going to do because of the chickens. This might be silly to some but since I've assumed responsibility for my chickens and I just won't leave it to anybody to look after them. But that's just me.
Best of luck going forward.