How about this..
Okiya is really okiya, a someone who is a fan of yusaku, maybe after conan knew that okiya is a fan of yusaku, he used that chance to talk with okiya sometimes while using his father voice.. then, he request okiya to do something for him.. since its yusaku, he just okay with it..
and okiya really dont know anything.. and I guess okiya really is the second brother, thats why he have some charisma like akai..
So, maybe akai told conan he have a brother who is a fan of yusaku, a like-detective man..then conan use the voice of his father to contact with okiya.. request him tk do a favour or something like that..
Akai always there but he is still hiding everywhere, even the second brother@okiya dont know about it..
And I really like the idea that the man outside of the house is not BO's.. conan knew that, and thats why he is okay to bring out akai..