I don't know what kind of grill paint they used, but I bet it was regular .98 cent Wal Mart spray paint. I've done more than a few guns with grill paint, and it once it has been heat cured, it has been very durable. I wish I still had the Mark I .22 I refinished, I'd take a few pics and post. I carried the gun quite a bit and shot it more than a few times, and never had any problems with the paint coming off. I don't know about the "doctored" jobs you are speaking of, but it certainly hasn't been an experience of mine.
Grill paint is very effective, expecially on guns you don't want to sink too much money on refinishing. Also, I don't know why rust would be a concern, as the finish is applied over the finish all ready on the metal. The grill paint adheres well, and leaves a nice matte finish when fully heat cured. There is no need to strip the finish down to the bare metal, so rusting isn't a concern. Of course, you still need to wipe the metal down with some type of oil, but that's the case with most all finishes. If the job is done right, there is no need to worry about refinishing it any time soon.
If you blue a gun with out plugging the bore you will definitely pit the bore and chamber which will result in having to polish the bore and chamber which will change all tolerances.
This simply is not true. I worked with a gunsmith for about a year, and he did most all the repairs in the southeast for Remington and Winchester. We reblued more than our share of firearms, and never once plugged the barrels. We used standard hot bluing salts, and never had a problem with rusting or pitting bores after the bluing was complete. Also, we didn't experience any accuracy issues. This man built some excellent shooting guns, all from in the white barrels and receivers that had to be blued. If there was any problem with not plugging bores while refinishing, I assure you this man would have known about it. You don't become a licensed smith for Remington or Winchester without knowing what you are doing!