You are correct that no new tourist visas are being issued at this time.
Your only option right now is to give up the M visa, and get as many extensions to the S2 visa as they are willing to give you. The only way to hold onto your M visa right now would be to return to your home country and wait things out (and it could be a while).
Once things cool down and China resumes normal travel, you can go back to your home country and apply for a new tourist visa, or try for the M visa again.
> what is the story with no more visa extensions for tourist visas?
Some foreigners were denied 30 day extensions last week, and were told they didn't show sufficient evidence there weren't flights to their home country.
Since then, other foreigners have been granted 30 day extensions. Not sure exactly what's going on here, but they have apparently not stopped issuing the extensions altogether.
> will 10 year visas still be valid?
If you're currently outside the country with a 10 year visa, it will be valid once the borders fully reopen to tourists, but it won't get you into China right now.
> where can we find the official government policy?
Good question. There have been official announcements about no longer being able to enter China with a visa issued before March 28, and about the blanket 60 day extension granted to most people, but there has been no national policy announcement about visas since.
Note that every province and municipality has the power to grant extensions and interpret the rules, so you'll see different situations around the country.
are visa runs still possible?
No. Most of the borders are effectively closed or greatly restricted, and people on tourist and other short term visas are not being let into China. This is why people with multi-year tourist visas are suddenly needing to apply for 30 day extensions.