of course no one on here can know what has / hasnt happened HAW.... but i'm going toi stick my neck out here. have been involved as professional in child protection cases for many years and a lot of what you have told us would be ringing alarm bells for me, viewing what you have told us free from your closeness to the situation. the more you have told us the more concerned i am for your dd.
she has made these allegsations to a virtual stranger, outside the family, which must have been v hard
she has rpeated them in a v credible way to the police in a formal i/v which must have been a terrible experience
she has not in any way retracted her allegations / changed her story
she has no obvious "motive" for lying such as a difficult relationship with new step father etc.
as far as you know she does not tell lies
she has given a detailed account of something that has happened a number of times (wouldnt a one off suffice for a made up malicious allegation?)
your h does drink. she says the abuse happened after he was drinking. he has lied about his drinking, in a formal police i/v
he had opportunity if, as dd says this arose when you were out at work and he was home with her.
have to say i know you keep writing of your confusion etc. but i really think you have to 100% get behind supporting your dd here for all the reasons people have posted.