I have somewhat the same situation at times but more focused on the expectations of command, control and execution. I have much higher expectations of my dog's performance than my wife's expectations...which has made me wonder if my efforts are being somewhat diluted by my wife's more forgiving expectations of the dog's performance. The thread is located here Command and control expectations
Much of the advice given by others was beneficial especially regarding your question " Will my hubby confuse the dog by suddenly changing the meanings if commands?".
I have also found that my use of hand signals tempers the problem you might be experiencing regarding two different verbal commands for the same action i.e. "drop" and "out". I strive to train the dog primarily with hand signals versus verbal commands once the dog has associated the two...so others can say whatever they choose and it does not dilute my hand signals even if it is for the same desired result. I would think it should be very easy for the two of you to sync up on the exact same hand signals regardless of whatever verbal command accompanies the hand signal if any at all.
Dogs are smart and obviously many breeds including a GSD are very smart. With that in mind, your dog will soon learn ( if not already ) who means what they say. My wife "hopes" the dog will comply to her request while our dog knows I mean what I say because I don't "hope"...I expect, plain and simple. Our dog's understanding of the differences between the two of us is simply that...we are two different humans with different expectations...and the dog absolutely knows this. Yes, I appreciate the frustrations which can be created by two individuals who are not on the exact same page when it comes to training the same dog but it is not something that is a deal breaker as far as the dog's education goes....but of course I would prefer, all those involved in a dog's training to be on the same wavelength.
"Should I be concerned about my hubby imitating play biting?"....I'm guilty of the same charge. When I choose to let the dog blow some steam off at times I have no compunctions whatsoever if we romp and she uses her jaws in a playful manner. However, when she used to get a bit more "playful" than my idea of fun...I would simply disengage the exchange. Let's face it, the set of tools a GSD has is much more formidable than some of us give them credit for....so I think the exercise all on it's own has value as the play biting can be taught to only reach a certain level which in turn creates a bite inhibition which is acceptable for playtime. My pooch like our others in the past, has figured out it prolongs roughhousing if she gingerly grabs hold of a loose sleeve or pant leg than using a tad bit too much bite pressure on a forearm...needless to say, a few of my sweatshirts have really long arms on them from her fun time. Granted, this type of interaction of roughhousing is my choice and most likely not the choice for all. However, when I am on the ground letting her have the upper hand ( in a toned down fashion ) and she has my sweatshirt sleeve in her jaws tugging away.....I do wonder what the neighbors might be thinking...ahhh but who cares.
Oh, by the way....my pooch is 18 months as well and it seems like a magical time as she is truly coming on line and now has such a focus on learning....it's truly amazing how quickly they learn at this age.