Answer: Hard tick (Ixodidae) nymph emerging from the larval exuvia. The inverted "U-shaped" anal groove on the ventral surface (photo below) allows us to identify this as an Ixodes species.
Thanks again to Dr. Graham Hickling for donating this fascinating video. Thank you also to Dr. Robyn Nadolny for the additional information about the function of porose areas and Gene's organ. If you'd like to see the Gene's organ in action (and tick eggs being laid), check out this other amazing video by Graham.