I agree that asking the questions that you have will get you apple and orange answers and that you should do what you want to do.
That being said I think that a lot of the "shoot it and forget it" responses are BS. How many of these guys actually live that way? Would they accept a less than perfect new automobile from the dealer with a shrug? No they would likely want it made right or accept the defect and a discount.
Your first photo is pretty poor, the second one of the mag well interior is likely white ink from a metal marking pen and it indicates that the part has been inspected by QC.
KAC's response as you have relayed it to us is really poor. The goofed up trigger is also as Mike points out really poor. I have purchased two ARs that were a third and two thirds the cost of a KAC and they seem to be able to manufacture, assemble, test, and the ship their firearms without having them blemished.
If you want to get rid of these white marks don't you a green pot scrubbers for sh!tsakes, they are abrasive. Simply get a rag and some naptha, varsol, gun oil etc and try to wipe the mark off. Use a green Scotchbrite pad and you will make a mess that you can't undo.