I'm not sure of what you mean by "the NEMA tests". All of our control boxes are UL rated and they do require initial testing. However, to my understanding, NEMA rated boxes do not. I could be wrong, but I thought NEMA ratings basically means they "would" pass the testing for any particular rating. Like I said, I may be mistaken.So maybe the parts guy is only guilty of bad grammar. That should be no surprise to this forum.
It is correct to say that NEMA 12 and NEMA 3R are completely different ratings, each with its' own criteria. I suppose, in theory, that it might be possible to make an enclosure that would pass one and not the other.
In practical terms, I can't imagine how one would make such an enclosure. I suspect that a little research will show that the enclosure carries both ratings.
The NEMA tests have virtually no consideration of the aging of gaskets, etc. There as some rather mild corrosion tests. Indeed, were I to try to make something that really was 'indoor only,' this is where I would try to do it - though that has become more difficult since the standards were revised to require even indoor enclosures to be galvanized.
Rain hood? Probably not necessary with that gutter flange. I'd go with the manufacturers' recommendations. Likewise, it's possible that the manufacturer would suggest the addition of a small 'weep hole' in the bottom face.