Here is a solution, but the procedure is not too easy to find:
To delete all cards of a (top) category without removing the category structure and without moving the cards to Uncategorized: Organize cards, Search, category, select category, Search Now, Edit, All, Delete, Delete.
To delete the structure of a top category with no cards: Organize Cards, tap-hold the category, Delete.
Most difficult to find for me: To remove all categories from a set of cards: Organize Cards, Edit, Batch, Remove from all categories. This procedure even allows to move cards to Uncategorized, which had been moved to "nowhere" (no category, not in Uncategorized) by Pleco before.
Tap-holding a top category with cards in order to remove the category structure does work for 1 (the first) of my 3 top categories. The second top category goes to Uncategorized (about 30% of the cards) or to "nowhere" (the rest). The cards of the third top category all go to "nowhere".
Another procedure which does not work is Organize Cards, Search, Category, Select category, Search Now, Edit, All, Delete, Remove Category - nothing happens.