Slowdown in Russian economy started 2013 , before sanctions. In fact due to sanctions , economic growth somewhat picked upward due to higher domestic industrial production to replace European products.
Oil is priced in US dollars , when oil price goes down and ruble depreciates they get more...
Actually, some in Ukraine thought Ukraine should give away strategic nukes but keep some tactical. It was US pressure that removed both.
You`re right , i mean great powers play their games but they certainly don`t want to nuke each other. They want to have nuclear free proxies :-)
No , they didn`t , Budapest treaty is memorandum which means there is no legal obligation by any of the signatories to enforce it. Ukraine got screwed by both Russia and USA over the nukes.