edit: I've reached out to Monit for an explanation, and will amend if they reply.
Something I've learned over the years is that, when it comes to legal issues, things are rarely black and white. In this case, whether or not Inspeqtr should be required to adhere to the AGPL probably comes down to whether or not it is legally a "derived work". And I'm going to go out on a limb and guess that there isn't a large body of case law covering the situation where a product is completely re-implemented in a different language. But it certainly seems within reason to say that a court could find that this is a derived work, and therefore in violation unless the AGPL license is used.
Now, whether or not this sort of thing is covered by the spirit of the GPL/AGPL is another question. But it probably is, if you consider the stance of Stallman and the FSF vis-a-vis Free Software. Remember, they basically consider all proprietary software to be a sort of immoral, unethical affront to humanity.
No. At most it means you might have to adhere to the license Rails uses, if your work is sufficiently related to be a derived work.
see Oracle v. Google.
If the reimplementation was made by perusing the original, it can be considered a derivative work subject to copyright.
Sure you could say "so and so downloaded Monit using IP address of 127.0.0.1 at this time", but I don't think that really is legally conclusive.
Unless the alleged violator actually posted public comments about "perusing" the original source code there's not a lot you could do. Unless you do discovery on all of his computers and he didn't delete the Monit source code. To go through all that trouble for a copyright violation on free, open source software seems hardly worth it.