why is self defence a red herring? surely that is the point of making aikido techniques effective- otherwise we could just apply them ineffectively and still get the exercise and other benefits. It is just we would be doing more in the nature of dancing than a martial art.
As I said, a real attacker is very unlikely to just stand there and wait for you to do something. So your ability to execute a technique in the situation you describe is almost entirely irrelevant to your ability to apply aikido in a self-defense situation.
Which, by the way, is one reason why that type of ukemi is only useful in certain limited circumstances.
Edit: Yeah, what Janet and Mary said.
Also what Pauliina said. Verbal communication is often more effective in getting your point across.