1) Why would you rule it out because it has a GIA report? Just because it's GIA? If the stone looks awesome, and has great angles, it's a contender. The reason I personally don't recommend a GIA report is because GIA doesn't truly grade cut quality. They put a "grade" on there, but their "excellent" means jack squat. AGS grades cut quality, which is why their grade means something.
2) Look at the numbers, look at the Ideal Scope and ASET images, and see if any of the stones really "do it for you", besides being within the budget. There's no need to go high in color, and get a clarity that is "eye clean to your definition" meaning what is eye clean to you. Their definition will likely mean face up head on only and from specific distance. My personal definition meas "can I see it AT ALL without my super loupe from any angle, any direction, in any lighting condition, PERIOD?" If so, then you cannot label that eye clean. I don't go below VS1 for that reason, but most "normal people" can do perfectly fine to SI1 or sometimes even I1. I cannot personally handle that. But I will snap up a nice J in a heartbeat. Bec the warmth is awesome--esp in a stud. It's on your head--which you cannot see--not your hand.
3/4) Regarding the Tiffany: They usually cannot pull reports on any stones below like .33 cts or something other than engagement rings. Solitaire studs are hit or miss, as are solitaire pendants. Regardless, there aren't exactly "set numbers" for anything outside of rounds. There are some working "ideas" for fancies, but nothing set. And at that size, not really helpful anyway since those don't typically get graded or viewed individually, they're parceled.
I am personally not a fan of that design (Victoria), so I will be biased in favor of a pair of simple studs, nor do I think the brand is worth it for something easily copied by just about anyone else. They do have some designs well worth the pricepoint, this just is't one of them. Further, the brand doesn't select stones for cut quality, they select stones for color, clarity and size--which is what their sales psychology is based on. "If he really loved you, he'd get you the biggest, whitest, clearest diamond we sold!" You will be hard pressed to find a good array of winners in their inventory, as many of the posters I have assisted on here with a Tiffany setting might tell you, BUT if that design speaks highly to you and having the branded item is important, by all means, get what you love.