for business or at P.O?
Affixers were owned by businesses.
The evidence of affixer miscuts/misfeeds in this era creates no surprise that many businesses soon switched to metered postage.
why would that cover with Fraser on it be double rated? Overweight, oversize?
Without contents, I agree there is no way to tell exactly. The stamps were not consecutive from the coil - perhaps they put one stamp on each outgoing letter that day then figured out that some needed another and applied them in a second round of applications. There was no "oversize" restriction back then, only weight.
(On the National Lamp Works cover) I suppose it's possible it was part of a postage due group and did not get marked because it was not the top cover
If due as unpaid with a return address, it would have been marked and returned to sender. Absence of any auxiliary markings would indicate a very high likelihood that it passed through the mails successfully.
Almost looks like it was cut with scissors
No, a knife-like apparatus. Like trying to tear a phone book in half, one feathers the pages ... to knife-through a stamp the device cuts from one edge to the other (in a scissor-like motion) or from the center to both sides, rather than cutting the entire stamp height at once. It often leaves a slightly diagonal or arced cut, the same on both sides.