1. Most of the core players for Classic show little to no interest in BfA. They could release it any time and not have to worry as much about that.
2. See 1. You may get a few checking it out, but it's more likely to be less than people who would want to try out Classic during a BfA content drought.
3. Now or Later probably doesn't matter much for Blizz. They are doing fine now, so later could work too.
And you are right about them not wanting to announce it without having a proof of concept/working alpha... but that just means they know it works. It isn't any real indicator of how far they got before the announcement. Or an indicator of if they continued working on it after they did enough to get the project greenlit.