First of all OP this is a good, detailed, and well-balanced review, especially for a new punter and reviewer. Sorry to hear it wasn't the positive first punt you hoped for. Let's take it chronologically.
I've no way of knowing whether the "family emergency" was true on a specific occasion, but Amber is notoriously disorganised and may have had a statistically unlikely number of such emergencies over the years. Accurate or not I imagine that the delay increased your apprehension / anxiety.
In paid sex a WG ought to adhere with the punter's wishes on reasonable requests, so she should've been dressed as you requested (assuming she confirmed that she had the relevant outfit). For future reference remember that it's fine to politely assert your wishes. In this situation you could have reminded her of your request, ask that she changed, and insist that the meeting starts from the time she reappears (you're not paying for her to change into something she should've been wearing anyway). Any attitude or refusal and you walk as she should've paid attention to the prearranged agreement and should be willing to rectify her oversight. This comes with experience.
I, and I suspect others didn't find her behaviour as you describe. Usually once the sexual activities begin Amber is an enthusiastic, responsive, and accomplished SP who throws herself into the action. We didn't include anal, but I rarely come and she had no problem with that or the prolonged action. In fact she seemed to relish it, or at least gave that impression. Within your well-written, honest, and balanced review you highlight the potential contribution of your inexperience and anxiety and that may possibly have influenced the interaction between you. I don't know how you came across to her and what effect that had on her, but the way you describe the situation that seems a possible factor. She's an experienced WG and may be expected to adapt, but that doesn't mean that she may not have felt vulnerable, awkward, frustrated, etc in response to your presentation. This is supposition on my part, although it is based on your description. I also assume that frequently changing positions in anal may be uncomfortable.
It's your decision how you rate a review. There are aspects of her behaviour as described that may edge a negative, but as you acknowledge the potential contribution of your inexperience, anxiety, etc and how those may have influenced your presentation and the interaction between you, a neutral may be appropriate.
Welcome to the wonderful world of punting, and glad you found your way to UKP. I hope that you use the forum to inform your future punting and that there are better times ahead.