Yes, if you manufacturer and sell 80% receivers, the ATF will come to the place of manufacture and give you its blessing. They inspect the receiver to make sure it is only finished to a certain amount. I don't believe it's exactly 80% as that saying was just made up somewhere along the line. There are a few things the ATF doesn't like when it comes to 80% receivers and pre-marked holes for the fire control group is one of them. I was told this information by someone who manufacturers 80% receivers and has the letter of approval from the ATF.
Yea, there is no set percentage the ATF deems nessesary in order to consider an 80% receiver a paper weight. They just make sure the remaining amount of machining requires a certain degree of difficulty. Marking the FCG holes removes some degree of this difficulty.