Hi,
-Regarding the first question yes,although I think it's also possible to put something like a pi-match passive network to match LNA output if you want to characterize it using s-parameters and a VNA.For the mixer you can use the same pads due to the fact you will want to characterize it's gain conversion using voltage gain,also take into account the need of a Buffer for the Mixer measurement.
-Yes the analysis you said are the necessary,for mixer measurements refer to:
http://www.eel.upc.edu/rfcs/Material/Cadence%202004/Mixer%20Design%20using%20Spe....
PS1:if you got a background in microwave engineering and want more information regarding the stability factor used in Cadence i did a little ieee search and found the following references(I'm curious about why it didn't use the classical definition):
Review and Discussion of Stability Criteria for Linear 2-Ports,RENE P. MEYS
Reappraisal of the Unconditional Stability Criteria for Active 2-Port Networks in Terms of S Parameters,DONALD WOODS
Stability and Power Gain of Tuned Transistor Amplifiers,Arthur P.Stern
Any comments regarding the use of B1f and K instead of delta and K will be appreciated.
Hope it helps, and sorry again for my error in identificating the different criteria used by Cadence.