@RichardN : it's not a single user but one customer (a company) who decided to enroll as a developer.
So I created an email for them, then the dev account. I developed the app and published it (on Android and iOS stores). After that, based on the initial agreement, I gave them the source and any useful account data. This is why they forwarded that email to me: I no longer have access to that email account or to the dev account.
Now, letting them to accomplish administrative chores (like the email verification suggested by Google), I will firstly try the "tricks" to move the deadline ahead or, failing, to modify just the Manifest.
If nothing works, I will need to take with me my notebook and find a place where Internet is strong enough to download and install anything needed for B4A 13. I know it sounds easy, but I'm leaving for a short vacation to a place where even phones work at random (yes, a real paradise, in this respect
)
When done, I'll post here to close the thread. Thank you all.
Edit: trick#1 (deadline to Novembre 1) accepted but I fear it only covers the time available to update the app; so I will also try trick#2 (Manifest change) hoping it will suffice to clear the risk of account deletion.
Edit#2: app published soon after trick#2 (Manifest change) was applied. The day after, any warnings about dev account suppression or policy violations were removed.