Obviously if Harper is in fact heading south to do some kind of negotiating he would have had much better optics if he would have discussed doing so with the man who currently has the power to sign agreements with foreign governments. All he's done so far is muddy the waters.
If he is seen as acting on Trudeau's behalf than all the disrespect the administration holds for Trudeau will inevitably affect their dealings with him. This way he's seen as an independent voice of reason.