User blog comment:Unmei Suiheisen/New player needs an answer about Platinum Exchange./@comment-26447088-20170209232040

She will not until after the next round of Elite Packs, as Platinum units in Elite Packs appear twice before being added to the Platinum Exchange a few weeks or so after. This is Ruprecht (Platinum)'s second appearance, so she will be added to the Exchange soon after the event ends.

Yes, she will be eventually available from the Platinum Exchange, and platinum medals have no expiration date.