Fallout – Is it possible to make covenant city a settlement by capturing it forcefully

I ended up doing the same thing, if you get the castle before Preston offers you the quest, it won't have any impact on the minutemen questline until you get to the point where he offers the 'retake the castle' (I believe you need to complete three or four settlement recruitment quests for him)at this point you get the speech option to say somthing to the effect of "i've already cleared that place out" and you automatically skip to the second stage of that quest, where you need to build enough generators to power the radio antenna at the castle. After that the minutemen quests proceed as usual. It actually saves time doing it the way you have.

Can't prove a negative, but I've never seen any shipments from the vendors that I build. I've stocked several with tier 4 general store vendors and they still don't have any shipments of materials.

As for Bunker Hill, it becomes a possible settlement point after a certain quest:

The Battle for Bunker Hill

And there are shopkeepers there that sells some shipment items. It seems to be the only possible permanent way of purchasing shipments while on a settlement.

Update:

There are a few other possible settlements with a npc vendor that sells some shipments:

Abernathy farm and covenant. However for covenant, you need to:

Side with Dr Chambers in order to not have to kill the town.