1) what aperture do you typically use when taking seacapes, mountains, fields?
f/11 to f/16 usually work best
2) I've seen a few interesting photos taken at F22 - is this advisable? Don't you lose sharpness & detail due to diffraction?
Yes, you do lose a fair amount of detail to diffraction, but some for some scenes it might be necessary. Focus stacking works better, however.
3) Where do you focus when there are no special focal point?
You can generally get away with focusing at a point 1/3 of the way into the frame - i.e. mountains or other background are 300 feet away - so you focus on something roughly 100 feet away. If you have close subjects like flowers, focus closer and use LiveView and the Depth of Field button to check focus, which is a good idea for all landscape shots.