Wow, this is a big problem...
If I have this right, you have no option for Google accounts and no Play Store. On top of that, either the Play Store will be blocked from access to their servers, or downloaded apps from there will be blocked. Is that correct?
Sideloading PS would not be an option, since it would be blocked due to a lack of certification anyway. And that's ignoring the lack of Google account, which would make the Play Store pointless anyway.
This means no option for adding paid apps, or apps which work on subscription verified via the Play Store.
Surely that means no Spotify or Tidal...no?
First option I can think of is to reach out to devs on XDA and ask if anyone would be willing to approach this issue, either by making 9 run on the new player or circumventing the blockages. But I'm not sure the circumvention would be feasible - plus it would put you in Google's bad books. And from your post above it seems that your software development would have to go back to square one to run on 9.
Second option is to reach out to Google and see if ANYTHING can be done. But I doubt you'd have much luck there.
Third option would be to approach it on an app by app basis and work with the devs to find an alternative form of secure authentication for subscriptions and verification of purchase. Some of the smaller teams might be up for this (UAPP for example,
might be) but the big boys probably won't. It's not in their interests, they won't be making additional income.
It's pointless to say "you should have just updated to 9". If the player has been in development for 2 years, you wouldn't have known in advance about Google's plans to shut down the certification channel. Once a project goes past a certain point in development, you can only go forwards with what you have. And to update to 9 at this point and reconfigure/rewrite the bulk of your software would delay release to a point where the product loses its place and relevancy in the market.
Talking of market, of course China will be a large focus for you and here the Play Store is not even a consideration since all Google services are blocked. Outside of China though, this is a strike against an Android based player.
People will see this as Fiio's problem, but I think it is Google's. More and more they are moving away from what made Android such a popular a widespread platform, and edging towards a more Apple-esque vision in terms of control and restriction.
@FiiO - I think trying to establish a line of communication with Google to seek a solution which can be put in place for your company (not just this one device) is probably your best choice of action to solve this. I don't know if you would be successful, but the real issue is that you currently do not have a route to certification, and that lies with them.
I'll be buying the DAP anyway - though it will be a shame to not have access to some other apps, I am first and foremost looking for a device to play my music collection on, and it will do this.
Good luck!!!