There is an edge case that the rules as written do not address.  I consider it probable that the United States will detonate a nuclear weapon on the territory of Ukraine, and blame it on Russia.  This would not be a "demonstrative detonation", but it would be on the territory of its "ally" Ukraine.

Ok, I apparently misread how you are using the word "demonstrative".  I interpreted it to mean a nuclear test.  The rules say a "demonstrative detonation" must occur in order to settle as yes.  I don't like the way this one is written, because there are a few edge cases that should settle as YES but the current rules would settle as NO.  Here are the edge cases:
1. USA or Russia fires a nuclear armed missile at a ship of the other in international waters, as a result of escalation of the Ukraine conflict.
2. The USA detonates a nuclear weapon on the territory of Ukraine or another purported ally in order to blame it on Russia, ie. false flag.  Because Ukraine is a USA ally, this would result in a NO under current rules.

Zelensky calling for preemptive strikes on Russian nukes. https://twitter.com/RWApodcast/status/1578076573223292929

Does a dirty bomb count as a "nuclear weapon"?

Apparently some people just died from a Russian missile in Poland
https://apnews.com/article/russia-ukraine-war-zelenskyy-kherson-9202c032cf3a5c22761ee71b52ff9d52

This market will resolve as YES if a nuclear weapon is detonated in the course of the current war between Russia and Ukraine by the end of the day of July 31, 2023, local time. The weapon need not be used on Russian or Ukrainian territory or by one of the direct combatants to the conflict, the nuclear weapon use must simply be directly related to the events of the war that began in February 2022. A demonstrative detonation by one combatant on the territory of an adversary will cause this market to settle as YES. A Russian demonstrative detonation on its own territory, the territory of one of its allies, or over international waters will not count. The same rules apply for a demonstrative detonation by the United States or one of its allies. 

Play-money betting