QFF
I will answer in reverse order,
c) Because if you use QNH for drawing meteorological maps the pressure lines will be influenced by local temperature and elevation - This is highly undesirable
QNH is not a "correct" pressure at MSL. The question should actually be asked in reverse - Why does the pilot then use QNH?
The answer is that the onboard instruments is "assuming" that ISA is present, therefore keeping to ISA in the calculations will avoid adding any temperature error between ground and MSL.
Simply (on the ground): The pressure at the airport is measured and MSL pressure is calculated according to ISA (QNH) and exact elevation. The altimeter also measures the pressure at the airport and you input QNH - The altimeter then uses ISA and calculates to find your altitude (in this case elevation)
It is the same calculation with the same values which means that the result should be correct and without temperature influence (except instrument errors)
a) + b) The isothermal lapserate is referring to the lapserate of the temperature:
ISA = 1,98 degrees/1000´ , ISOTHERMAL = 0 degrees /1000´
Explanation:
The ISA pressure lapse rate (27´/HPa) based on ISA temperature which also has a lapse rate. If you change the temperature the ISA pressure lapserate will no longer be correct. This is why you correct min useable FL for temperature.
Example coming up, hoping I didn´t confuse you already..