This note in [[DefineOwnProperty]] caused confusion during code view:

NOTE: Attempting to redefine an immutable value always fails, even if the new value would be cast to the current value.

Maybe just inline ValidateAndApplyPropertyDescriptor and replace https://tc39.es/proposal-immutable-arraybuffer/#sec-typedarray-defineownproperty, steps 1.b.ii-viii with:

1. Let mutable be true.
2. If IsImmutableBuffer(O.[[ViewedArrayBuffer]]) is true, set mutable to false.
3. If Desc has a [[Configurable]] field and Desc.[[Configurable]] is not mutable, return false.
4. If Desc has an [[Enumerable]] field and Desc.[[Enumerable]] is false, return false.
5. If IsAccessorDescriptor(Desc) is true, return false.
6. If Desc has a [[Writable]] field and Desc.[[Writable]] is not mutable, return false.
7. If Desc has a [[Value]] field, then
   1. If mutable is false, then
      1. Let current be TypedArrayGetElement(O, numericIndex).
      2. Assert: current is not undefined.
      3. Return SameValue(Desc.[[Value]], current).
   2. Perform ? TypedArraySetElement(O, numericIndex, Desc.[[Value]]).
8. Return true.