Hi! Congratulation
s on your beautiful pup!
The chewing gum is not a big deal, but in my opinion, stairs is. Carry please as long as you can because the damage can be created in early age for the whole life. Climbing up as I know is not so bad, but just for few steps, but climbing down puts to much weight on his joints so it is strong no-no. I see, you do not have much choise at present so carrying him is the only solution.
About the walks. Try to imagine life of this pup - he was comfy and safe under mother's wing and probably had not had used to outside world much. So strets, cars, smells - all seems completely new and yes, scary, so try to take him just out at the doorstep now, let him to get used to @his@ territory and then expand it.
On top of that, puppies usually have they scary periods, I do not remember, but probably somewhere at 12 -14 weeks or so, so do not be surprised, if he gets scared again - that will be the time when he will grow out of his puppy skin and will recgnize himself as a dog.
If it helps to you, here is my story; I needed to take my complete country pup at 15 weeks of age to the city for two weeks - he was scared to death and it took 3 days to get him do his business outside (before he was completely housebroken already, but now he dashed back and did it all on the newspaper in the hall), and walking along the main street was like worse torture for him! So many cars, noises, smells... After two weeks in city he was not keen to leave! And it does not affected his nerves and temperament at all - he has nerves of the steal and nothing can scary him - he just loks there with curiositylike @is there coming another nice surprise specially for my peasure?@
Puppy class is great for socialization reasons first of all, and it gives you good training too. As you may know, menthal stimulation can wear dog even more than physical excercises so really give him a rest after the classes. So an hour in class replaces a long walk and a lot of playing at home. Keep it in mind - if you have not much time for a walk and games, a bit longer training session even at home would replace it.
Do not worry about lash manners and niping - it is typical puppy beheivior and later will improve with a minimum of training.
With here or come command try to be always positive, and keep a treat as a revard for coming - never let you be upset as there will be not 100% response for a long time, and never run after a pup to catch it - always run away from him - I bet you know it already, but it is so easy to forget it when pup is running away. Call him, get his attention and then run away - he will follow you to catch you, and that is the time for the prise!
Good luck and I'm so envious - I must wait for my pup until Christmas just to be born! And then all the waiting to take him home! OMG! And you aleady have all the fun now!!! Lucky you!