> all other nations would 1) formally declare war on Russia 2) start conventional open hostilities on Russian forces to defend Ukrainian territorial integrity

There are six obligations in the Budapest Memorandum. Your two aren't part of it:

https://en.wikipedia.org/wiki/Budapest_Memorandum

https://treaties.un.org/doc/Publication/UNTS/Volume%203007/P...

Russia signed as well. Russia broke with their obligations when they annexed Crimea in 2014 and again with their further invasion of Ukraine in 2022.