2. My fiancee is in the U.S. on the K-1 visa I obtained for her, but I'm not sure I'm ready to get married. Can I extend my fiancee's K-1 visa?
No. The K-1 nonimmigrant status can neither be extended nor changed. If you don't get married within 90 days of the K-1 status validity period, your fiancee will have to leave the US. This is a very strict law in US immigration and there are no exceptions.
http://www.arctec.com/fianceevisa.html
なので、発行日から6ヶ月間の猶予のみのようですね。
After You Receive a K-1 Fiancé(e) Visa
(途中省略)
With your visa, you can apply for a single admission at a U.S. port of entry within the validity of the visa, which will be a maximum of 6 months from the date of issuance. You must marry your U.S. citizen fiancé(e) within 90 days of your entry into the United States.