So he created Yukiko as 'The Navigator' to fix the path which led to Ai's death. This is the element which exists in world B which did not exist in the real world, world A. Inaba being at the birthday party led to him gifting the hairpin to Ai and consequently he got into the university because Ai cheered for him. This is why he thinks this time in the virtual world things will go differently and why he wants to go back to the birthday party.
Yukiko Inaba I feel is the wild card which will lead to the next plot development from here. She clearly has a mind of her own. She also exists in both worlds, although since Makoto is in her real earth body it's not clearly in which form she exists but I feel she will let her presence be known. We also know from earlier chapters that she is conducting her own parallel research with which Old Man Makoto did not want to cooperate.
It is hard to tell where this goes but if I were to guess either Inaba is going to wreck things or the Old Man fucked up and things won't go well in world B.