If you use COMMAND mode, then your approach is now clear.
We just need to agree on the names of the various concepts.

We use the concepts of Group and Item list to define static tables with a fixed number of lines, each of which corresponds to an Item that is usually managed in MERGE mode.

On the other hand, if you subscribe to an item in COMMAND mode, then the item values carry commands that say: "ADD, UPDATE or REMOVE a line based on a specified key".
Hence, by subscribing to a single Item, the client can maintain a whole data table based on the updates received for that single item.
So, if you use items in COMMAND mode, you will usually subscribe to a table that, from Lightstreamer point of view, only contains one Item (i.e. the Group associated to the table is made of only one item).
When only one item is involved, it is not important whether you use a SimpleTableInfo or an ExtendedTableInfo; your Metadata Adapter will receive the item name and shoud pass it through to the kernel (note that the ExtendedTableInfo is to be preferred because it still simplifies the management of the fields).
On the client listener, all updates are referred to the unique item involved, but, for each update, you can extract the value of the "key" field involved. Using a HandyTableListener is recommended.

Please check whether your questions about groups are still needed in this scenario.