It may not be so intuitive, but you can define states which will appear in the drop down by hitting the enter button when typing in the "Values" field in the new UI. Using the latest released Mango, I was able to change a virtual binary point to multistate and back.
Indeed. Not very intuitive. But it does indeed work as you explain.
Can you share a screenshot? If I open an existing multistate point the save button is not disabled, and I don't need to change anything to be able to save it.
In the situation I describe, the save button is not disabled. Clicking it results in a "Please fix errors on form before saving". Hard to give you a screenshot, as the message clears itself in a few seconds.
Edit: To reproduce this, you would have to attempt to edit a multistate data point that has no start value defined.
Second Edit: How do I add screenshots without making them available on a webserver?