ad 1) Clear, but hard to do when you have no idea if or when you will meet one of them (except the Vault-Rep). And - do you know: Should the store be empty (i.e. you build a store, but never assign somebody until you meet one of them) or can a Settler be assigned to the store (he will be reassigned or fired for the level 4 merchant)?
ad 2) Clear, you add a settler after all.
ad 3) Where did you hear this? Who said it? If you have it from a wiki, where did this information originate?
ad 4) Why? What does 'more smoothly' mean? Either it works and the NPC shows up, or he is missing, after you hired him/her. I already managed to assign the Vault-Rep, but he has no special requirements according to the wiki.
And - you forgot to mention, that many of the level 4 merchants have requirements regarding the number of people in the settlement, including connected - via provisioner caravans - settlements. They are quite high, up to 30 settlers, which needs at least two settlements connected.
The problem with all this is, that nothing is documented well. So there are a lot of unfounded rumors and legends and all kinds of bullsh*t, which does not help.
One problem is, that you never can tell if and when you will meet them, and it may be that you *never* see them in game during 100 levels. I met the Scribe - just as I arrived in DC the first time, at level 10 or 11. I have never seen him again, not where I met him and not elsewhere. And none of the other NPCs. Maybe it is so by design, or maybe Beth trust too much in their encounter system. Encounters seem to dry up in a way, after you had around 10-15 different encounters in a place, no new encounters happen there. The Vault-Rep is the only one I know how and where to meet.
The major complaint one can read here and elsewhere is, that the NPCs, after they are met/found in the game can be hired, they agree, and never show up in the settlement. And this is not a problem with your requirements, I think. This is just Beth, who are known for systems and things which do not really work dependable. If a script tells a NPC to go from A to B, it is not sure the NPC will ever arrive at B. Even if B is only 10 meters from A, and there are no obstacles. Just take the time and watch. They do not walk a direct path - often they take detours, and sometimes straight into a wall...
I sometimes wonder how the Beth devs can live and be creative and make enjoyable games with such undependable subsystems. But that may just be me as an engineer...