Stacey creates personas around herself, as was referred to when Jean told the anecdote about Stacey dressing up as a Red Indian; she didn't just pretend to be that for a short period, she immersed herself in it. My theory would be that she creates these personas to escape her depression and sadness. There have been hints, certainly in Lacey Turner's performance (where she checked her make-up in Thursday's ep), but alas not in the often lazy script, that she is doing this to dull the hurt rather than because she actually wants to. It's therapy.
She has alot to be depressed about, Bradley is the tip of a very very big iceberg. The Jean/Sean conflict, the falling out with Ruby, caused by Sean and echoing the situation with Bradley. You look at Stacey's situation as a whole, and it looks even worse than when you look at each individual problem. Her negativity is self-destructive, but understandable.
Max has not slept with Stacey, rather a persona she has created.
Stacey has not turned into Kat, she is pretending to be Kat (if they make this permanent in a lazy bid to create a replacement for Kat then the EE writers will have ruined one of their best characters). If it leads to Stacey becoming more independent, then its good.
It's all about being wanted, and Stacey has created a persona which facilitates the objective of being wanted. When she befriended Ruby the need to be wanted ebbed away. If she were a "slapper" she would have carried on even after befriending Ruby.
Important to remember: Bradley never sincerely said "I love you" to her so its easy for her to think he doesn't, and for people to influence her thinking that way.
BTW I think Max was not remotely supportive of her during the pregnancy, we now know what the true motives behind that "support" were. His not informing Charlie and Mo about the pregnancy, and not going to fetch them after the bust-up with Bradley, suggested to me he only cares about his own family.
What Stacey needs is a new friend more than anything else (if Preeti is not going to be that then what on earth is her purpose on the show?).