I'm sorry if I seemed rude, I've been a bit grumpy today, so that may have come across in my earlier post ^^!
This is just a thought I'd figure I share, I'm not sure if it will help you or if you've thought about it. (Hey, I may be playing the role of Captain Obvious over here, but it can't hurt to say ^^).
If she said she would be willing to participate in a polyamorous relationship before and then she suddenly changed her mind, it's likely that her feelings towards you have changed. There's a fair chance that she really fell for you and really didn't like the thought of sharing you. Even if she loved you when she said she would do it, this is still a possibility.
I'll give you my feelings as an example. I am currently in love with two men. My boyfriend, we shall call him M and my ex, we shall call him T. I love M less than I love T, the love is also a bit different. If M came to me and asked to have an open relationship with me, I would be willing to give it a shot (at least in theory, in practice I might not be so willing after trying it). On the other hand if T and I were to start dating again and T came to me and asked to be in an open relationship with me I would not be so willing to do it and I would almost certainly say no. Although, I would still be willing to participate in a threesome with him and another woman or another man, it would just be this idea of him being with another woman when I'm not around that would upset me.
Actually, now that I think about it I guess it could also be that in theory it seemed alright to her, however in practice it didn't. Or that she didn't like you spending a week with the other girl alone. (Or another reason).
I think a good idea would be to figure out exactly why she said no, maybe it's something that can be worked around. Then yes, it's likely a good idea to figure out any other potential limits and work on a 24/7 Dominant/submissive relationship. (Unless all you want her to be is a housemaid now and not anything sexual, I mean if you're going to be all picky about vocabulary
)!
All right I'm done with my random mumbling ^^!