OK, now I'm missing your point. Jobs was out in March '85 when Sculley foiled his failed coup and subsequently stripped him of all duties. The fact Jobs didn't submit his official resignation until September was a technicality, he was already mentally and emotionally out of there, if not physically in an empty office building on the outskirts of the Apple campus. Of course the Plus case and analogue board is the exact same design as the 128 & 512 and the 512K was released in just about the same window from the 128K as the Plus from Jobs being stripped of duties. Either way, adding a fan to that case is a no brainer and as Aoresteen points out, internal upgrades were readily available by third party suppliers with no modifications required. As I pointed out, the analogue board was modified in 86 after the Plus was already released. Sticking a fan in the case at that time even would have been no more difficult for Apple than third party vendors. In fact it would have been easier. Also, Sculley asked Frogdesign to update the Mac Plus case just before the release – this was done completely without Jobs participation and eliminated features that Manock had designed and Jobs had previously approved. So in Jobs' absence, engineering a simple fan for the Plus release would have not been a problem either. So there must have been other factors.
My point is simply that putting a fan in the Plus at anytime, even a small one, would have solved many of Apple's headaches and in Jobs' absence would have been painless to do, especially when they were updating the analogue board to support the additional load anyway. The case did not need any special re-engineering or design modifications to include one. Once the SE was announced, Apple could have used the change to Platinum to add the fan to all of their models across the board, again with no major changes to the existing Plus configuration.