1)I would say yes to seperation anxiety but also you have been with her almost 24/7. Don’t give in to the barking and whining when you leave her. She will then do it just for attention and you don’t want that. She will learn with time she is still very young.
2)She is still small so playing with other dogs may be intmidating for her. It is good that she rolls on her back when they come up to her that means she is open and submitting to that dog saying your the boss to them. You want that rather than aggressive I don’t respect anybody attitude. My pit Kira was the same way…believe me they grow out of that now she’s the boss! Something I highly recommend is puppy classes. It socializes them very well. I would check with local training groups in your area and see what they have and when you can start her. It teaches them manners and handling by strangers (ie. Vets). Easier to start now than later. You can even do it at home. Play with her mouth look at her teeth open her mouth and make her do it and make sure she is calm when you do it. Also try grabbing her rough like a kid would do. Like grab her tail and things then it teachers her that it is ok when that happens and not to bite.
3.) I wouldn’t play tease or tug games with her alot it could make her more aggressive and I don’t allow mine to growl really aggressive when I play with them or when they play with each other. A little is ok but some times if one gets to mad it excalates and someone ends up yelping. So I monitor them when they play and stop it before it starts.
As for the potty training they are very good with that and learn very quickly. Walking off lead is good but as she gets older she will want to explore more and that can lead to her running off. Plus the waiting for you is good she is learning quickly you are the pack leader and she looks to you for the way she should act.