As for her final 50m, Rebecca Adlington has done a faster final 50m at the end of an 800m race (28.91 Shanghai 2011) and I don't recall the same comments when that happened.
...

Can somebody post some other historical cases of swimming personal bests being bettered by 7 seconds in a few months at the world class level?

Yes. American Janet Evans which a much larger improvement from 14-16 than Ye:
400 Freestyle: 1986 (14yrs old 4:16.74, 5th at the Goodwill Games), 1987 (15yrs old 4:05.40, WR at US Open), 1988 (16yrs old 4:03.85, 1st and WR at the Olympics)
800 Freestyle: 1986 (14yrs old 8:38.07, 3rd at the Goodwill Games), 1987 (15yrs old 8:22.44, WR at Nationals), 1988 (16yrs old 8:17.12, WR at Nationals and Oly gold)

For comparison Ye Shiwen:
200 IM: 2010 (14yrs old 2:09.37, ranked 1st in world), 2011 (15yrs old, 2:08.90, ranked 1st in world), 2012 (16yrs old, 2:08.39, ranked 1st, still more than 2 seconds slower than the 2:06 world record)
400 IM: 2010 (14yrs old 4:33.79, ranked 2nd in world), 2011 (15yrs old, 4:33.66, ranked 2nd in world), 2012 (16yrs old, 4:28.43, WR+Gold)

http://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Forum_F1/Ye_Shiwen_-_16_y.o._female_Chinese_400_IM_Gold_Medalist_P4074216-4