Nick had no reason to suspect it was Adalind. He would only know if he had access to the same information we have. Besides the south clearly won over the north in that situation. He just assumed Juliette wanted to have an afternoon delight and didn't question it which I assume plenty people wouldn't. There hadn't been an instance of anyone impersonating someone else at that point in the show.
Trying to explain this body morphing won't get us any closer to the truth. It was just a means to an end to eliminate Nick's Grimm. I don't know if it was to get Adalind pregnant with Nick's baby because that probably doesn't happen if CC didn't get pregnant. We can question a lot of things about it like what would've happened if Adalind got drunk as Juliette. Would she immediately sober up when she regained her original form? Would the smell of alcohol completely disappear from her breath? I have no problem accepting that whatever she did as Juliette stuck.
Trying to explain this body morphing won't get us any closer to the truth. It was just a means to an end to eliminate Nick's Grimm. I don't know if it was to get Adalind pregnant with Nick's baby because that probably doesn't happen if CC didn't get pregnant. We can question a lot of things about it like what would've happened if Adalind got drunk as Juliette. Would she immediately sober up when she regained her original form? Would the smell of alcohol completely disappear from her breath? I have no problem accepting that whatever she did as Juliette stuck.