There was a small amount of cheating in the original stint, after which the real Kat would not have cheated again. The first time was with her ex who Alfie owed money to, who had threatened to kill Alfie if she didn't sleep with him. Due to the circumstances, some didn't even regard this as cheating. Alfie did and was cold with her for months. Eventually she started drinking a lot which resulted in a one night stand. Alfie was going to leave so she left instead. It was fairly awful for the character, but was nowhere near the trashiness of alleyway guy and was excused by many as it was a plot device to write Jessie Wallace out for maternity leave.
While Kat was away, Alfie had a non-sexual relationship with Kat's sister, Little Mo. When Kat returned, Kat bent over backwards to win Alfie back. For a time, Alfie was seeing both at the same time (but didn't sleep with either until he finally chose Kat.)
Kirkwood never got anything right as far as Kat and Alfie went. Knowing how much Alfie and Little Mo hurt, she would not have slept with his cousin. Given how he reacted to her cheating in the original stint, he would not have been okay with her having cheated with his cousin (it was cheating because Alfie went to prison and they had agreed to meet by a fountain when he was released) and having said cousin's baby. Further, Kirkwood expected us to buy the fact that Kat slept with Michael because she was alone and scared in Spain or something. I don't think so, she has a daughter (Zoe) who lives in Spain who she could have gone to. Clearly the guys she was running from after that scam didn't know who Zoe was to make going there unsafe, as Kat returned asking for her birth certificate so she could assume Zoe's identity and continue hiding. After having made Kat have Alfie's cousin's baby, he continued to regress Kat by having her cheat further. He didn't show the maternal side of her that was so obvious in her original stint, even though she hadn't had the chance to raise her daughter. Instead we were expected to believe that she would leave her son's care to other people most of the time, after she'd also missed the first 4 months of his life.
As for the Derek Branning affair, I have no words. Ridiculous.
I loved the couple in their original stint and I love them now, it's just a case of blocking out Kirkwood's ruination. Obviously not everyone can, but I just think the actors have a great on screen chemistry which makes them so much more believable than other on screen couples. The backstory of each character was tragic in its own way (more so Kat's) and they contrasted nicely, as their personalities do now; Alfie is a necessary calming influence for her I feel.
Newman did a good job of redeeming them, although it wasn't great that Alfie kept things going with Roxy after realising he could never love her like he did Kat. I feel she could have ended the affair sooner, but that's the only criticism I have of them under her. Kirkwood had done too much work on the affair plot prior to leaving for Lorraine to quash it it seems, from an interview with some of the cast.
As for DTC, it's clear that his priority is Mick and Linda but the twins were an excellent decision by him to strengthen the couple. They've had more than enough trauma where babies are concerned and it is fitting for both of them; Kat who wasn't allowed to raise Zoe and Alfie who had to adopt a father figure role to his younger brother after their parents died, and who has brought Tommy up as his own. They've had their off moments (Kat's perjury/Stacey obsession and Alfie's schemes bordering on ridiculous some of the time) but they are currently the closest they've been to the couple who left. Not all of them obviously, but this does seem to have brought the couple's fans back (me included, I detested them from 2010-12.)
Alleged upcoming storyline however, big mistake I feel. We'll see.