No.
If TK is flying
IST-JNB-DUR-IST and not just IST-JNB-IST, it is because it wants to serve both cities. There would not be enough flow to fill that plane just from IST-JNB-IST or just IST-DUR-IST. This way they compromise by having a longer travel time, but providing higher frequency of service to both cities. Obviously they can't carry pax JNB-DUR, but that's beside the point.

The ET flight mentioned above is another one of these triangle fights, with both IAD-ADD and DUB-ADD on sale.

As I mentioned, the only flight with a PURELY technical stop is BA1. Even then, pax get to go through pre-clearance in SNN.

Is it a scheduled stop or does it depend on the weather?

PR's GUM stops are purely technical. They stop there solely for the purpose of adding fuel.

[humor]On how many balikbayan boxes are loaded.[/humor]

Yes, it's purely weather/loading related. All they do at GUM is add fuel.

Right but the 788 that ET uses for LAX - DUB - ADD wouldn't have the legs to do LAX - ADD (nothing in service right now would) so while they might sell rights on both legs it is a technical stop - they've just placed it strategically in an attempt to profit from needing to stop at some point.

Ditto with IAD - ADD at least when it was launched and using a 788 (which I don't believe has the range for IAD - JNB though these days they are using a 77L which might have the range depending on conditions since both ADD and JNB are hot and high).

This isn't as consistent as it used to be. The 77Ws no longer need to perform the stop at all; the A343s do it, but it doesn't seem to be done all the time.

Is BA 777's SYD-(SIN)-LHR a technical stop in SIN, or something you can book onto/off of at that point?

You can absolutely book BA on both the SYD-SIN and SIN-LHR legs. But I guess they are technical stops as that aircraft doesn't do SYD-LHR nonstop with reasonable loads.

Back in the '70s SAS flew CPH-BKK over Sovjet Territory with a DC-8, going down in Tashkent for refueling.
Everybody deboarded and was given a shot of vodka at the door to the terminal where you could buy Stolichnaya for $1/litre. After 45 mins back to the plane.

Kinda reminds me of UA's Island Hopper route through Micronesia, although those are not technical stops, just having many people who "deboard," walk around, and then re-board.

*disembark?