1st case : Irimi Nage
As for the way the body works, there is strictly no difference between such movements like irimi nage and ikkyo - which apparently look different .
2nd case : Ikkyo
The only difference is that, from an identical movement (same steps), nage uses in one case aite's rear imbalance and, in the other case, his front/forward imbalance.
3rd case : Kote gaeshi
The body work is identical again with kote gaeshi. The only nuance is that irimi nage is applied at jodan level while kote gaeshi is applied at chudan level.
Kote gaeshi must be applied low. O Sensei - Kuden
Aikido is not a sport, it is a martial art which laws (takemusu) are in harmony with the laws of the universe. Studying them allows the practitioner to understand his place in the universe. Aikido was born in Iwama, O sensei achieved in that village the synthesis of tai jutsu, aiki ken and aiki jo.
The International Takemusu Aikido Federation (ITAF) brings to the practitioner the structure he needs in order to work as close as possible to the reality O sensei MU defined. The official national representations are the guarantee of a teaching faithful to the Founder's.
In modern Aikido, weapons are hardly taught, if taught at all. In O sensei's Aikido, on the contrary, aiki ken, aiki jo and tai jutsu are unified and form together a riai, a family of harmonious techniques stemming from one unique principle. Each techniques helps understand all the others.
Peace is a balance between a human being and the world around him. The true martial art's goal is not to become stronger than one's opponent but to find in that opponent a way to realize harmony. There is no enemy anymore as such, but an opportunity offered to reach unified ki.