My Japanese is rusty, but I think you're talking about Ippon Seionage (sp?). Again, if you can pull those off, that's great. I was just always taught that any move that requires your back to be turned to your opponent is a liability. Also hip and shoulder throws can be tricky to pull off, and if you mess up, you're WIDE open to get countered.
No argument there.
You're certainly not wrong, and no, Judoka who cross train into BJJ do just fine. In fact, as all individuals who have grappling experience entering BJJ, they'll have an advantage over a white belt with no grappling experience, especially in regards to gi-based takedowns and throwing, and gi-based competitions. Hence why it's such a huge benefit for them to be entering into BJJ gyms, because eventually they become BJJ instructors and bring their knowledge of Tachiwaza with them.
Again, Judoka aren't dumb. They see what's happening, and they hate it as much as anyone else.
As for the Olympian who did well in MMA, that was Rhonda Rousey. She did fine until people figured out her gameplan (she was pretty one-note) and eventually obliterated her.
The issue is as time continues, you're going to see less and less Judoka entering MMA and BJJ competitions, and Judo is going to become an insular sport with little crossover appeal. It'll be a sport for people who just want to do Judo, not a sport for people who want to be better grapplers, or even for self defense.