1) I dont think If you select internal attenuator before calibration will give you wrong results.
2) In some spectrum analyzer there is a direct facility, they will display NF.
But in some spectrum analyzer you need to measure.
The actual formula for sensitivity calculation is
Sensitivity= -174dBm + NF + 10logBW + required S/N
This is commercial application notes from Maxim. Unless you need to measure exactly the same Maxim products think again before use this notes. Agilent's application note may be better for your particular application and it is much better technically.