Fallout 4 – How to Call for Minutemen in Far Harbor

I went and did the quest again but I forgot to grab screenshots. I could still build structures(defense turrets and walls) and I didn't have to re-establish my supply lines.

This happens. I've had an instance where there were definitely no enemies and in that instance you're just going to have to deal with it.

Essentially, explore the entire region around the settlement and see if you can find more enemies, if you can't, sit around for a bit and see what happens. If you have a siren then use it, see if the call to arms allows one of your settlers to see the enemies that are supposed to be attacking.

Settlement attacks are random rather than pre-determined which is why reloading the save didn't allow you to relive the attack.

Additionally, settler count and settlement statistics via the Pip Boy interface are not always correct, as discussed here in this question.