How were the coronations of the rulers of "new" kingdoms or empires (such as Charlemange or Otto I) arranged? Did guests attend with the knowledge that a new realm was being created, or did it come as a surprise?

No, they were kings in a long line of other kings. Charlemagne was the second Carolingian on the Frankish throne after his father Pippin. Before that it was the Merovingians for 300 to 400 years back to Clovis who was leader of a certain Frankish tribe before he conquered the others. Otto I. was the second of the Ottonians to rule East Francia after his father Henry. There was nothing exceptionally new about his coronation.