New house came with a huge unmanaged flock. Help!

[aart]

This is why I worry to eat them and I know they have never been wormed, inspected, or treated for any diseases over the years. If I feed them well for a few more months and give them diatomaceous earth mixed with their feed, do you suppose they would be safe to eat as long as they looked healthy at slaughter?

This will do nothing to 'cure' the birds of any ills (internal parasites).......
....but it will kill any grain mites, and maybe other pests, in your feed bins.

If they have lice/mites, you'll need more than DE to eradicate them, I recommend permethrin.
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).
Part the feathers right down to the skin around vent, head/neck and under wings.
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
https://www.backyardchickens.com/t/1134783/permethrin-spray

Ditto LG suggestion to do a close inspection of birds when you slaughter.
I save the intestine opening to look for worm load until last cause it really stinks.

Sounds like you have a pretty good handle on rounding up the birds.
You might consider live traps to get the more skittish.
Wire dog crates and/or a temporary enclosure to confine the ones you may want to slaughter or sell.

Where are you located in this world?

 

[bobbi-j]

I third the suggestion to examine the meat before you decide whether or not to eat the ones you cull. I wouldn't eat a bird that obviously looks sick, but a healthy looking one would certainly be considered. My favorite thing to do with tough old hens is to pressure can them. I LOVE having canned chicken available. It's so easy and versatile to use.