First - people need food, shelter, medicine and lots of other stuff. So working on the defenses has to be one of many projects. Also there’s no such thing as impenetrable, just ask the French. And of course all the projects includes people leaving the compound, which means they’re vulnerable then, so even if you get “impenetrable” for the compound the group is still vulnerable.
Second - the zombies aren’t that big a threat. There’s a whole host of better uses for their time, energy and ammunition than the zombies. One of my favorite Negan moments in the comics was when he came up to Alexandria to collect his tribute and found them shooting zombies. He was incredulous that they waste ammo on the dead, he said the dead aren’t a big enough threat for that, save the ammo for the real danger, people.
Third - you can’t put self sufficiency third. Your first and second require calories to be burned. Remember that’s part of why they took the trip to Hilltop, Alexandria was starving to death.
How in the hell are they supposed to know every blade of grass within 100 miles?! Here we have proof that you’re just basically talking out of your ass. You want them to know every blade of grass in an areas that’s 31 THOUSAND square miles ( https://www.google.com/search?q=area+of+circle+with+100+mile+radius&ie=utf-8&oe=utf-8 ), in a world that’s mostly traveled on foot, with no radios, no air coverage, and all with less than 50 people?! Really, think it through. Basically your entire post is a festival of “sounds bold and exciting, couldn’t possibly work in the real world”... basically you’re making election promises.
Yes, I was exaggerating when I said every blade of grass in 100 miles. My point -- hyperbolically made -- was that they needed to be situationally aware well beyond their walls. That way they can't be surprised, not only by attacks on their compound but ambushes when they're out on "missions."
Nobody said they had to waste ammunition on the dead. They have non-combustible methods for dispatching the walkers. With 50 people, they could each take out 20 a days and clear 1,000 from the vicinity. That eliminates one threat to the compound.
And yes, they'll have to eat while they're building their defenses, so not every resource will be devoted exclusively to defense. But it has to be Priority 1. It doesn't do any good to be well fed if you're dead.
I didn't say "impenetrable." I said VIRTUALLY impenetrable. The idea is to make penetration of the compound so costly any aggressor will consider other options.
As to the vulnerability of the outside missions, that can be reduced by recon and eliminating the walkers. If you have a good idea who's in your area and what their intentions are, you can either avoid them, defend against them, or eliminate them.
I frankly don't care if the whole stinking compound gets overrun by zombies with bad breath. I'm so sick of Rick and his band of boo-hoos that I'm cheering for Negan.