Here's the thing. You will never know what is best, indeed, there likely is not a best. There are only different paths and then different things that arise from them you cannot predict.
You write that your daughter is truly excited about this opportunity and seems, from your description, to have a good sense of herself for her age. What would be wrong with figuring out, ahead of time with her (with parental guidance) how and when this skip would be evaluated and what might be done if it is determined not to work? (i.e., can go back to regular grade, get extra help, etc.) Going into it with your plan to undue it before giving it a go seems a little premature...
I might worry what you signal to her, given the school's support and her own excitement, about herself if you say no based on fears. She has a chance to do something she wants to do, feels she can do and then what? No... because she won't tell you her inner thoughts? Some of which she might not even be able to articulate.
Also, one of your posts asks something like what if staying where she is gives her the best things ever. I think you also have to ask what if moving on does. She gets another year and will change/grow/learn either way. When I read that section, it seems as if you are feeling like you are stealing an actual year. You are not in either case.
