Hi nosambos,
Those little guys can slip through the tiniest openings.
They can come in overhead too.
Can you narrow it down at all?
I'm wondering if you can pay someone to come figure this out for you. They can carry disease and I would be worried about so many in your house.
I wonder.......some companies can do a thermal-heat check of your house to see where your leaks are. I wonder if that would help show little spots that the mice could enter through?
I really think you're going to need to hire someone to help you figure this out.
Do you have a basement?crawlspace?slab?

I didn't want to kill the mice either, but after so many were cought and released Hubby insisted. You need to worry about them chewing on wiring, we found several bare wires and who knows how many bare wires are now in the walls. I had been letting them go in my garadge in the winter.They are so cute and I knew they did not have a prepared winter shelter. I did not know they leave a scent trail and were just coming right back in. You could trap them , then cage them (feed and water) then when you have enough get them to a good natural wooded habitat far from your house. I will tell you I have NEVER had mice when I had a house cat.

Get a cat, let nature take it's course. If you don't kill them, or don't keep them as a so called caged pet, they'll keep coming back and breed more and more. Sorry, I'm not trying to disrespect your life choice, and totally understand not wanting to kill a living creature, but your choices are slim, unless you want to continue living w/ them as they multiply.