Although I can't speak for Ikeda sensei, since I helped translate that article from Japanese to English, I'll add my own thoughts here.
I believe ouyouwaza could very well include henkawaza in that in order to make a technique "effective," the basic form of the technique may have to be changed. This may mean that ikkyo may turn into an ikkyo variation or that ikkyo may turn into shihonage.
In terms of practice, I believe it depends on the terms of practice. Like you said, some folks frown upon changing the technique one iota from what was shown. Others would rather see nage's ability to adapt to a certain situation to effectively pull off a technique whether it's exactly what's shown at that time or not. From when I train with Ikeda sensei, I believe he leans a lot more towards the latter than the former.
As far as ikkyo to shihonage goes, one way to do this would be to to the initial kuzushi for ikkyo omote but make it "shallow" to make uke "bounce" back up. You can then use this bounce to go under uke's arm to do shihonage, usually omote as well...