Like I've said, you can stay in the starving riot-torn city if you like and good luck with that..
-
As for your other points-
1- Siting a hillfort on high ground gave the defenders an advantage because they could throw spears downward at the enemy, but with firearms a bullet doesn't need gravity to make it go faster. Anyway if Doomsday Hall was on a hill it'd be a sitting duck for enemy snipers..
2- After the apocalypse the Hall group will grab all they can from derelict stores before anybody else grabs it, such as guns, canned food, fuel etc.
Here the 'Survivors 1975' group are hand-cranking petrol up from an underground tank at a deserted filling station, then helping themselves to food at a deserted supermarket before the rats get it-
3- We won't have to "vet" new members, because the very fact that they want to join the hall group shows they're made of the right stuff and are the type we want..
4- There's an old military doctrine- "whoever fires first wins", so our rooftop sniper will squeeze rounds off at long range and the bad guys won't know what hit them.
5/6- Like i said, we're going to grab guns from deserted gunshops and food from deserted supermarkets rather than leave them for the bad guys to grab.
As for food later on, we'll grow/hunt/fish our own, here's the veg plot behind the Hall-