Okay admittedly I'm not certified in quantuam theory or anything remotely relating to such physics, BUT if dimenson CAN connect, and such powers can open portals (eg the Key), maybe the reason the key had to be opened at a certain time MIGHT be because of the harmonic/natural sympathic energies involved. Perhaps Dawn's "keyness" is related more to certain portals WHEN they are receptive to such energies. Perhaps now the Hellmouth WILL be receptive because it's opening at a certain time.
(btw agrees with Crothy that they couldn't just blam, take away her Keyness without some serious stuff being mentioned)
Perhaps the Key opens a portal, but it's only when the portal is at a certain energy peak. Much like tidal forces. The key itself doesn't open the portal, merely acts as a insert for a tumble lock. Meaning to open the RIGHT portal, you need the right things in place. Thus Dawn's keyness isn't lost, merely less accentuated.