Hi, I'm no expert but I think the questions you raised are definitely red flags. I agree the serial number looks fishy. Shouldn't there be three letters followed by four numbers?
My new oxblood Pepe had the same color zipper inside and out. Same with the leather tag. The tag inside is also oxblood. Also, this color looks rusty red in the picture. Not sure if it ever was "oxblood". Also, Did Givenchy ever make "oxblood" Pepe with gold hardware? I don't know. I only just found Givenchy last year. But all the oxblood Pepe's I've seen have silver. So that makes me suspicious too.
Authenticate4u I've used twice. The turnaround was terrible. She had a family issue so basically ignored her customers for about a week. No auto reply explaining the situation or anything. I would not use them again. But I don't really know who else to suggest.
Actually, if you're willing to be patient, I'd wait to get a new one on sale. It'll probably be not much more than what you're thinking of paying for this one. Check Farfetch every day! Every day. There are pandoras that pop up here and there (good colors too). Right now there is a beige goat small that's 30 % off. Sometimes they'll go 40% off. That's how I got mine. 40% off isn't impossible, it's very possible.
As for this one, I would honestly stay away. Just my opinion.