It is of course difficult to be sure from this distance, but reading between the lines, I think it is just possible that the stepmum is actually being quite reasonable. And, possibly, is being very wise. And that possibly your DD is (understandably) protesting this.
Of course, I say this because it all sounds totally familiar to me. I was this stepmum! Sounds like she is trying to restore some healthy boundaries and maybe get DD back into a 'daughter' role. ('spousification' is very real!)
I would have done, and did, the same. We went on holiday - DSD wanted to share with DP - I wasn't having it (and I do think it is highly inappropriate at that age to share a bed). Not to mention the other horrors of holidays together - the illnesses faked to get DP's attention, the bossiness, the insistence that we did what DSD wanted, ate when DSD wanted, talked about what DSD wanted to talk about...I spent every holiday in misery, for a while.
I had to have the same conversations about 'linking arms' - because while that is the way your DD reports it, I can tell you that I was on the other end of a full-on clingy campaign with my DSD at that age! - if DP and I were holding hands, DSD would literally push in between us and break our hands apart, then latch onto DP...so I may be biased, but I suspect there is more going on here than the stepmum banning her from simple arm linking with her daddy. Likewise with the business of coming downstairs after bedtimes. My DSD was on a campaign to dominate and have DP's full attention, and that meant interrupting on any pretext any time we were alone in a room, ever. I can't overstate it - it was deliberate, and it was unhealthy, and it was damaging to not only my relationship with DP, but DP's relationship with his son, who was constantly sidelined as well.
We are a couple years on now, and things are much much better - and actually, DSD is a lot happier! But we did set and keep those limits and she fought against it. And you know, I'm so glad we did, it was absolutely the right thing to do.