Re the JJK drop in form...
Like most heptathletes, JJK set her PB aged 26 (most seem to set their PB between 24 & 26). JJK was also at her peak then, those years between 86 & 88 were when she set those surreal marks of 7158, 7128 & the crazy 7291. So from a peak perspective, she is typical of most heptathletes. What makes her unusual is the obviously crazy scores, but also her drop after 88 as mentioned. Her 1991 heptathlon - her first since the crazy 88 WR - was a massive 413pts lower. It's not unusual to drop a loft after a PB; most top women have, but a drop of over 400 pts is the biggest. If we look at the last top women:
After her 7013 score (aged 24), Thiam dropped by roughly 200 pts for her next 3 heptathlons, before getting back to 50 pts within reach of her PB.
Jess Ennis set her 6955 PB aged 26, but then DNF the next year, then had a year out for a baby. She returned some 286 pts lower in 2015, before clawing back another 100 pts the next year, to be around 180 pts lower than her PB.
Carolina Kluft set her first 7000+ score aged just 20, and the next few years she dropped by 50pts, 100pts, 260pts (injured at Europeans..) and then beat it by 20 pts aged 24. But then of course she changes events at her peak!
Personally, I believe Kersee heavily doped JJK leading into Seoul, then purposely took her off in 89 & 90 to allow her body to recover. She hardly competed during 1991 (back on the juice?) in a build up to Barcelona, her last great hurrah in the heptathlon, and likely still juiced, but with a very different drug regime.