I can't test this myself at the moment so i'm wondering if anyone else knows if it's possible.

Can you just ignore Preston in Concord at the start and still build settlements? I know there's some you can acquire by just clearing out the hostiles and then activating the workbench, then building a radio beacon.

Which brings me to my next question - without Preston to tell you to go help settlements and ally with them, is it at all possible to just kill the settlers there and acquire it that way? (working the same as clearing out 'hostiles')

Well, maybe not as many.
Sanctuary and Red Rocket are your no matter which way you play it.

Railroad gets one busy building "Settlements/Safehouses" at its conclusion too.

I do not know about the other 2 factions, yet.

I doubt killing the settlers will "unlock" the Settlement Crafting bench.

I haven't reported back to preston at all.. and every settlement I come to, they themselves ask for help without Preston having sent me there. Of course I am part of the minutement so I don't know if that changes if you don't align yourself or not. I'd assume that they'd just ask for help even if you have no minutement ties but I could be wrong.

Edit, beaten to it by a few seconds.

Usually if you head there without being sent specifically you are given a quest by any settlers there. If it is empty, ie no settlers, then just activating the workbench after clearing hostiles in the area should work. At least that has been what worked for every random settlement I stumbled across.