Short-term visa rules stated in Denmark website

In order:

1. The paragraph is a bit confusing, it seems to reflect article 9 of the Schengen Visa Code (“Holders of a multiple-entry visa may lodge the application before the expiry of the visa valid for a period of at least six months.”) and the relevant part of the Handbook for the processing of visa applications and the modification of issued visas but goes beyond what's really in this regulation. Nowhere in the official text is there any mention that the new visa would be an “extension”, extensions are for other, very limited purposes.

2. I don't see anything in the official regulations implying that entering on a visa and leaving on another is something that would be limited to some visas but not others. My understanding is that if you do have two back-to-back visas without gap, you could do that. But since single and two-entry visas should be issued for specific trips, it shouldn't happen, which is why it's mostly relevant for multiple-entry visas.

3. The 90/180 rule applies in any case and at all times to you as a person. If you earlier visa is a type C Schengen visa, then it can't grant you more than 90 days of stay in the Schengen area in any 180 days. Your new visa does not change anything to that either. You can stay 30 days on this visa but in any case not more than 90 days at once. Also note that it's not 90 days during the period of validity of the visa, it's 90 days in any 180 days.

Also note that a two-entry visa is not a multiple-entry visa so Denmark did not have to accept your application for a new visa early. But if it did issue a visa, the point is moot. Incidentally, you got very strange visa. It seems highly unusual to grant such a long two-entry visa and then revert back to a short one-entry visa.