Google v. Oracle seems to suggest that you could do that, potentially? (IANAL.)

On the other hand, the fact that Arm doesn't produce chips might be an important factor, if there's an argument that the Java API is incidental to Java IP but the ARM ISA is the substance of the IP.

I think this would also depend on the extent to which the ARM ISA specifically is protected by Arm's patents, since Google v. Oracle was strictly related to copyright. It might be the case that what you're suggesting is plainly illegal, or I could see it being a kind of grey area where it's fine per se, so long as no one actually puts it into practice without purchasing a patent license from Arm.

What IP covers the ISA?

Is the ISA itself patented, or just the implementation? Or do they claim copyright over it?

Suspect that ISA can’t be patented but efficient implementation of aspects of it are. (IANAL again)

However the cost of implementing the Arm ISA in a clean room is probably more than the licensing cost from Arm - they are spreading their costs over lots of customers and you are not - so why would you do it?

Patents on all the efficient ways to implement said ISAs?

Exactly. In the past ARM has been excessively paranoid about cloning, to the extent of even threatening students.