A neat workaround if you want to add editable tables in Google docs containing merged cells.
- Make the table in Word or Excel, Sheets or Presentations. Do your merging. Then copy the table. In Google docs, go to Insert, Drawing. (I bet you wondered why Google had this feature that forces you to create a drawing on the fly... Here is why.) When you paste your table into the Drawing window, then click "Save and Close", the merged table is embedded in your Google Doc as though it was created within the doc. But what is really neat is that anyone with edit rights on the doc can double click and edit the contents of the table.