The answer to your question one: you calculate the average ISA deviation from TOD to landing, by averaging them. Isa at TOD +20, ISA deviation at destination +10, so +15 degrees. This will influence your TOD, your VNAV Path and fuel consumption. Or fuel remaining at destination if you will.
Question 2 I will not answer. The answer is simple as well, but you do not want my explanation, you want a reference. Google will be your best friend.