You can have only one and always derive other from it. Storing it twice is a waste.
Using timestamps in distributed network?
Yes. Same as in the present blocks. Where is the problem?
Bitcoin timestamps have +- 2h accuracy ;]
You cannot rely on it. Also person making payment have now way of knowing when his transaction will be included in block so can't calculate account balance after tx. There can be multiple payments to same account submitted simultaneously.
Foreign currency account without any way to link them to real world accounts?
Yes. With a link to "normal" banks, of course. Would be just a nice
additional feature.
There is no way blockchain can check validity of such 'links' so it is totally useless. You can as good write such links in your notebook or post it on website.