From all I've learned of Russia (including a class on imperialist Russia in university), a lot of this stems from the idea that, publicly or not, they still consider Ukraine to be a part of russia (as well as most other baltic and eastern countries). Your point on the access to the black sea also rings true to me. Russia Is heavily interested in a "Warm Water" port, and while the balck sea isn't the best option, it's currently the only one they've got. It's also a big reason why they keep messing with the middle-east (not just for the oil). I wasn't aware of the treaty. What happens if they violate it? are there set-out consequences?
I misunderstood the classification. It's actually not a formal treaty, but a diplomatic memorandum. So, if Russia does violate the agreement, which is to respect the sovereignty of Ukraine's borders, then it's illegal according to international law, but there's no real way to enforce the agreement. Since Obama is already saying that there will "be a cost" I imagine that the consequences are still up in the air. Haven't read anything from the UK's PM either, so I guess we'll find out later.
I think it's all a little bit of countries playing chicken at the moment. No one wants to make the first move and get all the blame if things go totally pear-shaped.