We are bringing in open transaction to a small subsidiary in our Oneworld environment. Is it "dangerous" to date them correct but select current period as posting period?

For ageing if you want to use original date and use period as current period then it all fine. Have done it before and see no issue with it.

that should be fine: foreign currency exchange rates are the things you need to watch out for. Should be mirroring the original rate on your open items (and remember to post the fx reval reversal in your first active period)