I'll attempt an answer.
There doesn't seem to be anything wrong with this per se, given the Death Note rules as I remember them. If sea monsters (or aliens) existed, this would be hypothetically fine as long as the conditions for such scenarios coming into play (e.g. A's journey to the sea doesn't take 24 days, or A doesn't have to fly from Urumqi to the Atlantic Ocean in an hour) are appropriate and possible. However, we run into some problems.
Obviously if sea monsters exist where A goes diving and the biology of sea monsters allows them to swallow A, we're fine. But how about aliens? What if, as I mentioned, it takes more than 23 days for them to arrive on earth?
If the aliens show up and kill A, or if the sea monster swallows him, we have proof of those things existing. But if A just dies of a heart attack, we don't know what we've "proved" or disproved. For example:
Maybe aliens (or whatever) don't exist.
Maybe it's impossible, at least at the moment, for aliens to show up and kill A within 23 days, because they don't have the appropriate technology.
With kaine's comment - maybe the mathematician simply isn't capable of solving the Riemann hypothesis that quickly for whatever reason.
So if we get a "positive" result, we're good. But if we get a negative one, we probably won't know why exactly such a result occurred, and in that sense this technique doesn't quite work (or rather, it wouldn't be as useful as one might imagine).