Lee, I'm so glad you're continuing to like Tipping the Velvet! When you're done reading it, I highly recommend renting the DVD too.

A little bit about Helen...
You are right. She is a likeable enough character... even though she is a source of pain for Margaret.
We know that they must've loved each other a lot when they were younger. But Helen 'grew up' and decided that it was time to live more 'conventionally'.
Remember when Kitty told Nan: "Can't you see? We couldn't carry on as we were.." before going off and marrying Walter Bliss... Same kind of deal here. Margaret was still very much in love in Helen. We can see that. It didn't help that Helen was married to her brother and was forever in her life, never allowing Margaret the space to completely move on.
Yes, I agree with your interpretation of Helen, and the parallels between her decisions and Kitty's. I feel like Helen and Kitty probably represent the life circumstances for lots and lots of historical lesbians. The fact that they give in and get married probably accounts for some of the reason that lesbian history seems so invisible sometimes. And, I still think there are lots of ways that the Helen/ Kitty type characters present a parallel to an Ennis-type character... in getting married mostly out of a concern for convention and security.
Margaret seems like another major category of historical lesbian... the lonely, mature woman who would probably have been called a "spinster" in some circles because she's older and refuses to get married.
How do you think Selina figured out that Margaret was a lesbian? With Margaret's prim and proper exterior, I'd think it would be really, really, really hard to tell.
Thinking of Margaret's image... I absolutely loved her costumes. Just gorgeous. And, one of my favorite images when it comes to the contradiction of the loveliness of Victorian women's clothing and the way that clothing was completely confining... is the scene where Margaret tries to climb that ladder quickly. It's interesting to see that she can climb the ladder successfully, but it looks really difficult in all those skirts, etc.
So, I have another question about Margaret... she seems to have a pretty good capacity for being "sneaky"... figuring out how to be left alone in the room with the prisoners' belongings... figuring out how to run back to Selina's cell for a quick final word towards the very end, etc. What do you think this says about her? It seems a bit like an anomaly compared to her overall image.