Hi Pete,
Using an 978MHz antenna is unlikely to result in damage to your PAW, as the nominal matching impedance will still be 50 Ohms, but as Lee has said there will be a significant frequency mismatch, so the 978MHz antenna is likely to be significantly poorer in terms of signal transmission (and equally so with reception). Can you not continue with the standard (attached) 869.5MHz antenna until the external ones come back in stock?
Sorry, I can't help with availability dates, but if you send an email to
support@pilotaware.com I'm sure Ash will be able to advise on expected deliveries.
Best Regards
Peter R