Wow - THAT was quick!
Lots of agreement with what you said, BUT:
Non-democratic nations should NOT have a presence, either formal or non-formal. in the UDN. One of the main purposes is to draw a bright line between the democracies and dictatorships of the world. The UN is flawed because it lends status and legitimacy to dictatorships, which are simply gangs who have seized power in a country. Of course, as a matter of practical reality, UDN nations will continue to deal with such nations separately, but without the imprimatur of the UDN.
While you are correct that democracy can be difficult to define, the definition should be made and incorporated in the charter. To prevent a gradual definition "drift" that ends up over time with almost any nation eligible, the definition can only be modified with say a supermajority of 80%.
That no non-democratic governments have a right to exist should be part of the charter.