Message-Signaled Interrupts (MSI) vs INT on PCI-E GPUs

Question

I've got a few questions about PCI-Express interrupts, because there are some misconceptions on the web regarding them. Specifically, I'm interested about PCI-E GPUs and interrupts.

What others on the net say about the matter:

1. Message-Signaled Interrupts are ALWAYS used on PCI-E because PCI-E simply doesn't support any other interrupt method - true or false?

2. This contradicts the first question: AMD GPUs use MSI by default, while nVidia GPUs use INT (INT being the older interrupt signaling method) - true or false?

My questions if actually some GPUs still use INT:

3. How is it best to determine if MSI is active for a PCI-E GPU?

4. If it's not active - how is it best to activate it: I know there's an app that does it MSI_util_v2.exe (is it safe?) OR manually in Windows Registry (and how)?

5. Are there any trustworthy benchmarks of PCI-E GPUs using MSI vs INT?

6. Is there a good way to check by myself the performance of a GPU using MSI vs INT?