But what you're saying is beside the point: Russia claimed the treaties were no longer valid because the new government isn't legitimate, yet still tries to invoke these treaties when dealing with Ukraine's debt. If what UR did was strawmanning, this must be too... Not that I think either post was, mind you.
IS that what Russia claimed? My understanding of it was that Russia has acknowledged that the non-Crimean parts of Ukraine are still "Ukraine" and subject to all the treaties with Ukraine, but that the Crimean parts of Ukraine have broken off and formed their own country. Because Ukraine (the non-Crimean part) will no longer be able to lease the military base to Russia (what with it being in the newly formed country of Crimea now), Russia feels that it shouldn't have to hold up its end of the bargain.
Remember, under the Partition Treaty on the Status and Conditions of the Black Sea Fleet, Russia is permitted to maintain up to "25,000 troops, 24 artillery systems (with a caliber smaller than 100 mm), 132 armored vehicles and 22 military planes" in Crimea. Wikipedia informs me that Russia currently has elements from one mechanized infantry battalion, two air infantry divisions, an air infantry brigade, a special forces brigade and a reconnaissance regiment in Crimea. So it seems completely plausable that Russia has not violated that agreement and that the Russian forces in Crimea are allowed to be there under international law. Further, given that all three of the Ukranian casualties thus far were caused by Crimean rebel forces rather than Russian forces, it can't really be said that Russia has attacked any Ukranians.
With those two points in mind, let's take the treaties one at a time:
4.All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations
This requirement seems to have been met so long as the Russian military did not use force to attempt to influence the secession vote. Russia is not using force as no Russian soldiers have shot at any Ukranian soldiers, nor (to my knowledge) has Russia threatened to do so.
According to the memorandum, Russia, the U.S., and the UK confirmed, in recognition of Ukraine becoming party to the Treaty on the Non-Proliferation of Nuclear Weapons and in effect abandoning its nuclear arsenal to Russia, that they would:
Respect Ukrainian independence and sovereignty within its existing borders.
Refrain from the threat or use of force against Ukraine.
Refrain from using economic pressure on Ukraine in order to influence its politics.
Seek United Nations Security Council action if nuclear weapons are used against Ukraine.
Refrain from the use of nuclear arms against Ukraine.
Consult with one another if questions arise regarding these commitments.
Points 4 and 5 do not apply here as nuclear weapons are not involved. 3 was possibly violated in 2009 as 10ebbor10 pointed out, but can probably be justified as a reasonable response to Ukraine not paying for any of the gas rather than an attempt at influencing Ukrainian policies. #2 and #1 are covered above in the first paragraph. #6 only applies if Russia is unsure about any of the above, which does not appear to be the case.