Does the scenario require Britain to capture Heligoland?

Does this scenario involve someone taking Heligoland from Germany?

No. Britain keeps the island and never transfers it to Germany in 1890.

The viewer's question clarifies the point of divergence before the detailed history begins. No invasion or reconquest is needed: Britain simply declines to transfer Heligoland to Germany in the 1890 agreement. That distinction matters because the island can then be fortified gradually as an established British possession rather than seized in a later crisis. The rest of the scenario asks how Germany reacts to a British fortress already sitting off its North Sea coast.