I saw someone say that spawns will not happen "on floors." So if you wanted to make Hangman's alley really safe, you could try to cover up all the ground with floors and see if that helped.
I haven't had much problem with the mob spawns. They are aggravating in that they occur inside walls . . . but another way of looking at it is that maybe they had some capacity to scale the wall or sap it.
I've taken a very different path on teh settlments the last 100 hours or so: basically just get them setup 'enough' leave them alone, and then go back once per week or so to check on them. In the span of a month in game time, I've seen maybe one settlement that got attacked which I failed to help with and one other one that I did help with. Actually, a lot of times, just as I show up there is a little mini-combat but it ends very quickly.
The main problems I've faced with settlements are: requisite to have the Tier 4 Strongback perk in order to haul everything around; need to carry lots of junk to places to manually scrap it so as to have plenty of build room when it comes time to expand from the very basic setup that can handle up to 15 but cramped.