Would be something of a holy grail to me to be able to "manage" data in M9, and have updates, inserts and deletes successfully pushed to a REST endpoint.
Very often, people who operate REST servers make sure you cannot do that. They don't want outsiders writing to their database, and they often don't even want to give access to the database so you can scrape the data from it. Most internet map servers are set up to be display-only by default.
The classic architecture for web servers that display maps is a two or three part ensemble:
1) All the actual spatial data is in a database like SQL Server, Oracle, PostgreSQL, etc.
2) An internet map server of some kind displays content from that database on demand, usually rasterising it for display purposes (rendering it) as .png or .jpg or whatever tiles that are sent to browsers. Turning vector data into raster data that is no longer tied to a GIS coordinate system is an effective way of showing a picture of the data without letting people get at the actual vector data for their own GIS use.
3) (optional) People will use a GIS package like Manifold as a client to connect to the database, to load, update, delete, and otherwise edit data. That client connects to the database either using native database drivers or ODBC or some other database connection technology. It doesn't normally connect back through the internet map server.
In your case, the way to use 9 to manage data within databases (which some internet map server publishes to browsers) is to connect directly to the database of interest. See the Databases topic.