Regarding the battery door, does the battery not fit under one of the back covers that came with the generic fat batts?? I often hear Seidio (or maybe it was Mugen?) back covers aren't as well received as the generic ones anyway, so if you stick with the Seidio you could always see if you still have one or buy another generic fat battery on the cheap and I could end up reviewing that instead or both, your call.
Amazon is likely hit or miss on anything that doesn't have Prime enabled (aka a highly trusted partner of Amazon themselves) and it's give or take the same experience as Ebay where you should realize that after a certain price or lower, there's no way it can be genuine OEM (and profitable).
Should you go slim once more, my offer stands on testing your Seidio, it's ok if it's got some miles on it. Do you maybe have an email invoice or something that can tell you just how long you've had it, for I always insist on including when a battery I review was purchased and for how much. (also whether or not you send it my way, I'm curious all the same at how long you've had it and what you paid originally)
I'm sure I could get a part # or sumfin hit me up if interested 
