Aren't you mixing ISA with implementation here? Or do Apple's CPUs really contain Arm's logic contrary to public info eg on Wikipedia?
If Nvidia bought ARM and decided to find some legal way to terminate the contract with Apple out of spite, Apple would have to find another ISA for their "Apple Silicon".
(Or, of course, Apple are making a huge mistake. But seems a bit less likely to me.)
One big point that came up in the congressional hearing the other day was how Google, when buying DoubleClick, said (under oath to congress) that not only would they not merge data but that they legally couldn't if they wanted to - and then years later did just that.
Is there any way to acquire in such a way that Apple would own ARM but there'd be a complete firewall between them, with ARM having a separate board, CEO, etc. and nothing between them except the technical ownership (and any contracts between the two companies)?
I hope not, as I'm in favour of breaking up huge companies myself... but if legal firms can have a system in place for firewall of information between clients I don't see why a similar legal situation could be feasible for allowing Apple to buy on the condition that they can't have any say over operations, with selling ARM being their only way of influencing them in any way?
Of course, owning a company where you have no control at all isn't great, but in this case it might be worth it if Apple trusts ARM to keep doing well without Apple's help, and if it would prevent someone like NVIDIA from shutting Apple out.
And would there be any antitrust issues if Apple bought a 500 year license to freely (or at a pre-set calculation of pricing) use any and all current and future ARM designs?
ARM has previously shut down open source software emulation of parts of the ARM ISA.
For a while QEMU could not implement ARMv7 I think, until changed their mind and started permitting it. There was an open source processor design on opencores.org that got pulled too.
The reasoning was something like "to implement these instructions you must have read the ARM documentation, which is only available under a license which prohibits certain things in exchange for reading it".
Third parties have been able to get IOS running in an emulator for security research and Android(!) has even been ported to the IOS devices that contain the "Checkrain" Boot ROM exploit (though with things like GPU, Wi-Fi etc in varying states of completion)
(The silicon implementation of an isa is referred to as the microarchitecture btw)