I just edited my response because I noticed it didn’t completely respond to what you wrote, so some of what you just wrote is in response to something I took out! Suffice it to say that it can be a hard decision to determine when/how to disclose trans status and leave it at that. And I agree that only that trans person can determine when is best for them.
I agree these are complicated issues and there are no right answers. I still want to reiterate that I appreciate you as a cis ally. I know what it is like to second guess things I did years ago. I think you will never know why Marla reacted that way. I will say, that I think trans women (especially early in transition, like Marla was), can be very sensitive. Losing friends and family is hard enough, but transition can be grueling and dispiriting (and, unfortunately, shows you who your real friends are), and you were nice to her. It is easy (at least for me) to mistake friendship for something else.
Thanks again for your responses,