I thought Yoh was having a temporary memory lapse because of his head injury. He may remember more as he heals. As for Aaron taking Fei, I have the (possibly far fetched) theory that Fei's brother Yanzui may be working with the Russians to get his revenge on Fei.
Yan is dead, he was shot in the head. That is very much canon from Rakuin novel. Yuri on the other hand could have survived.
Feis brother is dead though as far as we have read in FNR, yuri might be alive and might be involved in the kidnap of fei for revenge but this whole theory doesn't make sense, because they'll have to kidnap a kihito in order to get revenge but maybe kidnapping fei was second best to none since they had a lead on him but I don't know.
I get this vibe that yoh is withholding some info on who actually took fei plus Aaron taking fei (because that where are right now) doesn’t make any sense, since he knows now that fei isnt asamis weakness it’s takaba! So some of you might be right to ask who really is Aaron boss or who is he working with and him taking feilong could just be business related.