You can sign a pre-agreed contract before you are 16, it just doesn't become effective until you turn 16. That's exactly what Cairns did (presumably), and what Rushen has done.
If Oxford had adopted that model, running in all three league might have worked. Part of the problem was the exact opposite of that. It was very weird watching Sam Masters for Leicester, Charles Wright for Redcar, Jordan Jenkins for Ipswich, etc.
That's the Speedway Star getting the wrong end of the stick I'm afraid. It wasn't a direct interview with the Star, but a piece written from a press release. Said press release never said 'top flight operation', but simply that the Spires wouldn't be running in 2026. Fact is none of us know which league the Cheetahs will be in, so any speculation is exactly that at this stage... speculation.
Oxford a prime example of that. Cheetahs got bigger crowds than the Spires this season (not that the Cheetahs won anything, but they were at least more competitive).
It's not exactly just a speedway thing, though, is it? Happens in almost every sport.
Declan Rice (football, Ireland and England) and Israel Folau (rugby, Australia and Tonga) are two high profile ones I can think of off the top of my head... although I'm sure there are thousands of examples.