I did notice over the years various press articles about the 747SP which turned out to be (maybe) inaccurate - possibly either hopeful
PR people or journalist misunderstandings.
The original Pan Am purchase was described as for their round-the-world flight, where it was stated that the Transatlantic sector needed an aircraft bigger than a 707, while the operation onward across Asia needed something smaller than a 747. In the event I don't think the SP was ever assigned to the round-the-world.
The different aerodynamic relationship between the back of the upper deck (original 747 length) and the wing leading edge was "discovered" to give an unexpected benefit, and led to the 747-300 and -400 having the upper deck extended to get the same benefit. I always thought Boeing were pretty good with aerodynamics, and can't quite relate to them suddenly discovering this after the event,
The Qantas SP was indeed originally said to be for serving Wellington. Qantas (international only then) was moving to an all-747 fleet, their last 707 ran Wellington, after which they chartered an Air New Zealand DC8 to cover their rigorously controlled 50% of flights on the route. There had likely been criticism they didn't use their own metal.
American bought the ex-TWA SPs for their Dallas to Tokyo route, but ended up using them on the very short Boston to London route they acquired from TWA. American were really short of intercontinental aircraft when they first bought the TWA London Heathrow network, and seemed to use one of everything for a while.
The SAA SP fleet were really just an insurance against losing their landing rights at Sal on European flights when the Portuguese colony got independence. In the end this didn't happen before the -400 came along, which could do without it anyway, and SAA, like other operators, ended up at a bit of a loss for what to do with them..