Ok I did some more experiments. The conclusion is that the behaviour of columns in Odoo studio is very unpredictable. 

Examples:

1. I can only drag certain fields to another column when there is at least one other field already present in that column. Expected behaviour: I would like to be able to drag fields in an empty column
2. Only certain fields show the drag icon, others only the select icon (hand).
3. As mentioned above: no ability to add columns
4. No ability to re-order columns
5. No ability to add more than 2 columns via the UI

It would be great to see this column element improved in a future version.

Yes, Studio has always been quite frustrating to use. I find that often it's necessary to go into the XML to tidy up, or to do things that Studio makes difficult or impossible.

I have many thoughts on this here: https://odootricks.tips/odoo-studio/

Great addition, thanks