IMHO, the most effective way would be like this:

**For the 1st case:**

Method 1:

![enter image description here][1]

Method 2:

![enter image description here][2]

Method 3 (more effective for continuous editing):

![enter image description here][3]

**For the 2nd, 3rd and 4th case:**

Select target area, then <kbd>I</kbd> to inset, uncheck **Boundary** in <kbd>F6</kbd> panel. 

![enter image description here][4]

![enter image description here][5]

![enter image description here][6]


  [1]: https://i.sstatic.net/IzldJ.gif
  [2]: https://i.sstatic.net/fFRLr.gif
  [3]: https://i.sstatic.net/rWEJl.gif
  [4]: https://i.sstatic.net/uTnIB.gif
  [5]: https://i.sstatic.net/5UafR.gif
  [6]: https://i.sstatic.net/NeUgL.gif