If there are no functional alternatives, you could wait and see if they make progress or somebody starts with a server for the Unity client. Or you could also start with a new client yourself, but with the time left that's not an option for your retro. You could start with it anyway if you have time.
What I guess is gonna happen in short term is that soon retros are gonna make you install an alternative browser until they can make use of the new client/alternative ones, so I would wait until near the death of flash and then display a warning about needing an alternative browser, for now.