Well, after many years of thinking about and tinkering with this specific aikido problem I suppose I must accept one thing - ikkyo can be one tricky technique. Fascinating, though.
After one final session around the problem last night at my club we agree that much can be done to stop the discussed "reverse roll" out of ikkyo. Certainly I am a bit too nice in my general ikkyo training and certainly I am a bit too "pushy", actually encouragning a roll unless uke knows he's supposed to end up on his stomach.
A reverse roll does not necessarily put uke in a better position - so there's no real problem, one has to be aware though that a reverse roll is not always possible to stop - in strict ikkyo. The roll, however, can be easily stopped by going into other techniques, e.g. pulling back on the elbow and going into nikkyo.
In "pure ikkyo" a reverse roll becomes more difficult if one pulls back on the arm a little bit when uke is starting to go down. Also - force more straight down rather that forwards increases the likelihood of uke's head becoming trapped against the ground and therefore precluding any roll - or at least guaranteeing a true "pain in the neck". Being really "violent", basically slamming uke down on his face with the speed of light and the weight of an elephant, may be an effective way to stop the reverse roll, but it is bloody difficult to test realistically
Any future suggestions for how to carry out the perfect non-violent, soft and round but controlling, relaxed but failsafe, traditional aikido-certified and completely awsome ikkyo??
All the best