"On 21 February an agreement was signed between Yanukovych and leaders of the parliamentary opposition, witnessed by representatives from European Union and Russia. which promised return to the 2004 constitution, early elections and withdrawal of security forces from the center of the capital."[0].
2. Crimea was assigned to Ukraine in 1991. It's part of Ukraine's internationally recognised borders. It's illegal according to international law.
It was taken by force[1] without the will of the people. Any referendums under occupation are illegal and invalid (also according to international law)[2]. You can't vote with a gun to your head.
[0]: https://www.bbc.com/russian/rolling_news/2014/02/140221_rn_y... [1]: https://time.com/19097/putin-crimea-russia-ukraine-aksyonov/ [2]:https://www.gov.uk/government/news/the-crimean-referendum