If at any point in the future she decides she wants to open her end of the relationship, what will happen?

It depends how far along we are and where we're at emotionally. Let's say the emotional momentum has continued - I don't think I'd be too happy with it so we'd probably be at an impasse and have to stop seeing eachother or at least take a break. FWIW while she was thinking it over she asked if she could still see other people while she decided and I said yes, I wasn't super happy with the thought though I didn't tell her that and when she said that she hadn't seen other people I was quite relieved. We've also talked about having a FFM threesome, but that's more an intangible probably quite distant thing right now.

If at any point she asks you to close your side, what will happen?

It depends when kind of like above, if it's like 2 months from now then probably we'd have to stop but let's say it's 2-3 years then I think I'd be ok with that if I felt I'd mostly lived out what I was feeling I missed in my early 20s. At the moment I'm really happy with her and my emotional and physical needs are totally met.

This arrangement seems like it's intended to be temporary, is there an end game of when you will be ready to close it a be fully monogamous together? Or on the other side of the coin, a point at which you will be ready to have a fully open relationship?

I think long term I would want monogamy from a life partner, I've done casual before but for something serious that's ultimately what I would want. For her I think it's the same, though she hasn't had any kind of FWB or longer term casual thing she's not a stranger to casual sex and I know she's been in a few monogamous relationships. For a serious long term open relationship, I don't think that's really for me.

All that said it's still early days so this is dealing in far hypotheticals, we're not super serious yet though fairly emotionally invested.