Yes, many times. Usually through conflict or by treaties.
Borders
No
All countries share borders, although these may not always be land borders. Australia, for example, shares no land borders with any other country, but it does have maritime borders with other countries.
Any country that borders a sea coast.
The European Union has 27 different countries so there are lots of borders in it.
Spain has borders with Portugal, Andorra, Gibraltar (U.K.) and France.
No. Ottoman/Turkish borders were changed as a result of World War I in general and the Treaties of Sevres and Lausanne, but not as a result of the Armenian genocide in particular.
Jamaica is an island and so it does not have borders with any other countries.
Uzbekistan is not just landlocked, it's one of only two doubly landlocked countries in the world: not only does it not border any ocean, none of the countries it borders borders any ocean.
Oklahoma is a state it borders no countries
The Philippines does not share any land borders with other countries. It is surrounded by the sea and shares maritime borders with countries such as Taiwan to the north, Indonesia to the south, and Malaysia to the southwest.
North Dakota is a state and does not contain any countries.