> Are there any guidelines on when GIL should be released?

The GIL should be released:
- for CPU-heavy external functions (e.g. compression, cryptography)
- for external functions which wait for I/O

> Re PyTuple_SET_ITEM...yes that's also a possibility but it would then
> hide genuine bugs.

Well, as long as your monitor only increfs the tuple and doesn't mutate
it, there shouldn't be any problem. We use PyTuple_SET_ITEM in many
other places.