Szczepan, you keep on insisting on what O'Sensei, Saito, Shioda, Chida, etc. do/did is wrong, don't you? It's ok. End of the matter for me. I'm not trying to convince you feet apart is t3h 0n1y w4y, but you keep insisting all these people are/were wrong. Amazing.
Thinking that you are able to do the techniques the same way like O Sensei, Saito, Shioda, Chida did, is a frightening illusion. We, mortals, we are simply not the same level. So you can't copy the forms of such masters directly, from video or pictures. This is particularly true, in the light of the fact, that every one of them developed his own unique pedagogical system.
So the question may rise -- what ‘normal' aikidoka should do in such horrible situation? Where to find the points that will lead our own development?
Surly not by looking for differences between the forms of these masters. Rather, by trying to find some common points in their forms. One of such common point is unbalancing, other is leading after unbalancing.
O sensei was able to unbalance his attacker even before contact -- so ukemi of his uke looks like they are almost jumping over him. If you look carefully at attached pictures in this thread, you can see that his uke landed far away from him. What was the reason?
After all these observations, ask yourself: can I do koshinage exactly as O sensei did?
If answer is YES! -- keep feet apart as O sensei did.
If answer is NO! -- don't copy, but use your intelligence to search how you must use your body to reproduce similar result. So you will discover THE way to use your feet correctly (feet together?), who knows? If I may suggest -- just to be sure that uke will not tanking, do these experiences with not very cooperative uke that has more experience then you in aikido.