I thought there would be since there is a 64 bit edition for Linux
It's because you can't run 32 bits programs on a 64 bits Linux. Or, at least, it may not be that simple as in Windows.
Can you tell if this lack of ETA is due to no-one actively working on it
Nope, it's because this is something new that needs to be tested and reach a stable state before go to the stable channel.
is it safe to assume that it will eventually reach stable channel?
It will, most probably. But it may take a long time.
And, is the developer one compatible to the stable?
It's not advised to copy from stable to developer. Best option would be to enable Sync on both.