There are some simple steps you could take to minimise some of the problems quite easily.
I have been guilty of treating her as a mini adult at times
Can you give an example? She is 7 years old, there is no way she should be treated as an adult. She does not have the mental or emotional capability to process thought, information and actions, the same way as adults do, so treating her like this is can be very damaging to her. Stop it. Completely. You be the adult and treat her appropriately.
at times my discipline has been inconsistent which I'm improving on
Set the house rules. Talk them through with her. Tell her that you expect her to follow the rules or there will be a sanction - time out will do if you can't think of anything else, but ask her what she thinks a suitable sanction would be, you might be surprised at what she suggests. The point is that it happens every time she breaks a rule. When she follows a rule, praise her and give her positive attention. Don't give her attention for breaking rules.
her problem is that she feels that some rules don't apply to her and she is (a bit like me) not responsive to being TOLD what to do. She back chats, which my GF has no tolerance for - whereas I'm an explainer and happily explain WHY something is being done.
This needs to change. She doesn't need an explanation, that is just delaying and arguing. If you don't sort this out now, you will have trouble communicating with her when she is a teenager, regardless of whether you are with your GF or not.
It's not so much about discipline as teaching. You are teaching your dd the acceptable way to behave so that she can function and be successful in society. Teach her that back chat is not acceptable, in fact it is rude and disrespectful. You are the adult, she is the child, she should do as she is told (providing of course that you are reasonable in your expectations).
She does huffs, tries to hold my hand all the time, wants to come in bed in the morning to cuddle me, some attention seeking
Wanting to be physically close to you is not attention seeking, it's normal. Give her all the cuddles she needs. If it's not a good time, give her reassurance and be firm that she has to go back to her own bed but have lots and lots of catch up cuddles at a more appropriate time.
Hope some of this helps.