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hole? you mean for a lanyard?>  if you mean that small hole at the back, its for a lanyard

there is a cutout at the front strap so you can "r i p" stuck mags out more readily *use your forefinger) ; knida helpful   w/ the 1st gen mags that did not drop free   due to alack of a metal liner (rounds in em expanded the p lastic, rounds out undid it)

what gap do you mean? you mean the backstrap hump? if so, that is the way glock designed his grips/conours; if you are refering to the open hole back there, no, that is there to aid, again, in removing stuck mags (put your thumb in it and rip)––very help ful for stuck maga due todirt, mud, etc and/or simply  b/c they are stuck in malf

hope that helps

also, congrats on a glock

ETA: i dont know if these are "official" design features, but thats what i've always been told they wree for, and they work

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What is the hole for at the bottom of the back strap?,
Lanyard hole.
why is there a cutout on the front strap at the magazine well?,
Emergency magazine extraction/non-drop free mags.
is there supposed to be that much of a gap between the back of the magazine, and the back strap?,
Yes. The back strap channel does many things.
1)Allows water and debris to exit the trigger mechanism housing.
2)Attachment of a lanyard.
3)Attachment of a shoulder stock.
4)Access to the optional Glock lock.
5)Emergency magazine extraction.

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but I don't think that I've ever seen a Lanyard on a Glock.

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What are the numbers on the magazines, and guide rod?  They don't match the serial number, are they parts numbers?