There’s actually not a way to do this in standard Markdown specs as is; syntax for alignment is done on a column basis. See here
Rows can be added every time you hit return on a filled cell right now. Not sure how you’re creating tables right now, but if you type in the Markdown, it’s really easy to define the columns upfront. I agree though that adding a row-by-column interface to the button is advisable!
If you start typing the syntax as
| Head1 | head2 | head 3| etc. |
| - | - | - | - |
and then hit return, it will start a table with four columns for you.
Agree that something like a table of contents would be good. On iOS, the Quick Nav fulfills this need, but there’s nothing comparable on macOS. Actually inserting a table of contents might not be the most desirable. Maybe I’m wrong though. When I’m handling Markdown documents with a table of contents plug-in, I’m not always in love with the results.