Aussie
What you say about not descending below the circling minima until a normal descent at normal rates can be made is right - but only at night. By day, you can descend if you maintain the required obstacle clearance depending on aircraft category - see the quotes from AIP above. But - yes - you must remain clear of cloud, with the required visibility and keep the threshold in sight.
What you say about conducting a missed approach if an engine nacelle (or something else) prevents you from keeping the threshold in site is also correct - technically. That would rule out a circling approach in just about everything I can think of though, so I guess most people are applying a little bit of "commonsense" to the rule. Legitimate question though: how much "commonsense" before you wind up in trouble.
The important point about circling approaches, though, is that they are higher risk than just about anything else we do. And controlling the aircraft by reference to instruments and [some] visual reference is gonna be challenging ...
Ted